What is sodium nitrite

Sodium nitrite is used in many industrial processes, in meat curing, coloring, and preserving, and as a reagent in analytical chemistry techniques. Sodium nitrite is used therapeutically as an antidote in cyanide poisoning. Sodium and potassium salts of nitrite and nitrate (E 249-252) are authorized as food additives in the European Union. They are used in meat, fish and cheese products to hinder microbial growth, in particular to protect against botulism, as well as to keep meat red and enhance its flavor. Nitrate is also found naturally in high concentrations in certain vegetables, and it can enter the food chain as an environmental contaminant – mainly in water.

Nitrate and nitrite are naturally occurring ions that are part of the nitrogen cycle and are ubiquitous in the environment. In addition, since the early 1900s, nitrate has been used extensively in agricultural activities, mainly as a fertilizer. The content of nitrate and nitrite in vegetables depends on the type of vegetable, the method of production, the use of fertilizer, the season and light. Nitrate and nitrite are also used as food additives in processed food as preservatives and color fixatives in meat, poultry, fish and cheese ¹. Levels of nitrate and nitrite may be determined in raw commodities or in food as consumed.

Nitrate is a potential hazard because it can be reduced to nitrite in vivo and in food; nitrites can react with amines or other nitrosatable substances present in food to produce N-nitroso compounds ². Transformation of nitrate to nitrite during storage, especially in home-prepared food, is known to occur. Evidence from studies on methemoglobinemia support the contention that the nitrate contained in vegetables is converted to nitrite before consumption ³. Levels of nitrite up to 400 mg/kg have been reported in vegetables that have been damaged, poorly stored or stored for extended periods and in pickled or fermented vegetables ⁴. In contrast, levels of nitrite in cured meat appear to decrease during storage as it is converted into nitric oxide. The decrease is mainly due to its reactivity, and may average 70% ⁵.

The Joint Food and Agriculture Organization of the United Nations (FAO)/World Health Organization (WHO) Expert Committee on Food Additives (JECFA) ⁶ has evaluated the health effects of nitrate and/or nitrite and confirmed the previous acceptable daily intake (ADI) of 0–3.7 mg/kg body weight per day for nitrate ion and has established an acceptable daily intake (ADI) of 0–0.06 mg/kg body weight per day for nitrite ion, on the basis of the NOAEL (no-observed-adverse-effect-level) of 6.7 mg/kg body weight per day for effects on the heart and lung in the 2-year study in rats and a safety factor of 100 ⁷. This acceptable daily intake (ADI) was also endorsed by the Scientific Committee on Food of the European Commission ⁸. However, it was noted that these acceptable daily intakes (ADIs) do not apply to infants under the age of 3 months. Bottle-fed infants under 3 months of age are most susceptible to methemoglobinemia following exposure to nitrate and/or nitrite in the drinking-water ⁹.

Acceptable daily intake is an estimate of the amount of a substance in food or drinking water that can be consumed over a lifetime without presenting an appreciable risk to health. It is usually expressed as milligrams of the substance per kilogram of body weight and applies to chemical substances such as food additives, pesticide residues and veterinary drugs.

The current acceptable daily intakes (ADIs) for nitrite, set by the European Commission’s former Scientific Committee for Food (SCF) in 1997 at 0.06 milligrams per kilogram of body weight per day and the Joint Food and Agriculture Organization of the United Nations (FAO)/World Health Organization (WHO) Expert Committee on Food Additives (JECFA) in 2002 set the acceptable daily intake (ADI) at 0.07 milligrams per kilogram of body weight per day (mg/kg body weight per day). For nitrate both bodies set the acceptable daily intake (ADI) at 3.7 mg/kg body weight per day.

Nitrate

The experts were able to derive an acceptable daily intake (ADI) for nitrate as they did not consider it to be genotoxic or carcinogenic (for substances that are potentially damaging to DNA or may cause cancer no safe level can be established). The European Food Safety Authority panel considered the most relevant effect for setting a safe level was elevated blood concentrations of methemoglobin, caused by nitrite converted from nitrate in saliva ¹⁰. Based on this effect, the panel concluded that the ADI set by the Scientific Committee for Food (SCF) (1997) was sufficiently protective of public health.

Nitrite

The European Food Safety Authority panel calculated an acceptable daily intake (ADI) of 0.07 mg/kg body weight per day, corresponding to the safe level established by JECFA and close to the slightly more conservative current ADI of 0.06 mg/kg body weight per day derived by the Scientific Committee for Food (SCF). As for nitrate, this is based on increased methemoglobin levels in the blood following consumption as a food additive.

As regards the exposure to nitrates and nitrites, the International Agency for Research on Cancer Working Group made the following specific recommendations:

• (a) Infant dried milk preparations should be reconstituted only with water containing low levels of nitrates. If such water is not available, breast feeding or the  use of cow’s milk should be encouraged.
• (b) Only vegetables with a low nitrate content should be used in the preparation of baby foods. If vegetables known to contain high levels of nitrates are used, appropriate food processing precautions should be instituted. Nitrates and nitrites should not be added to baby foods.
• (c) The use of nitrates and nitrites in foods as preservatives should be reduced to the minimum level that provides protection against botulism. This applies particularly to cured and canned meats and to fish. The use of nitrates and nitrites on fresh meats or fish should be avoided.
• (d) Nitrate levels in public drinking water should comply with, or preferably be lower than, the tentative limit of 45 mg/liter recommended in the International Standards for Drinking Water.

With respect to the carcinogenic risk from exposure to N-nitroso compounds, it is prudent to assume that any exposure may involve some degree of risk, and that exposure should therefore be kept as low as practically achievable. This may not be an easy task in many instances, since these compounds may occur in the environment in concentrations of the order of parts per billion, as a result of a variety of natural and technological processes, and, moreover, they may be formed in vivo from nitrates, amines, and amides, which are ubiquitous. Obviously the recommendations (a) to (d) will also contribute to the reduction of carcinogenic risk related to N-nitroso compounds.

The European Food Safety Authority’s Findings on Nitrosamines

Nitrites – including when used as food additives – contribute to the formation of a group of compounds known as nitrosamines, some of which are carcinogenic. Applying a number of conservative (i.e. worst-case scenario) assumptions, the panel concluded that the formation of nitrosamines in the body from nitrites added at approved levels to meat products was of low concern for human health ¹⁰.

The European Food Safety Authority panel further noted that nitrite unintentionally present in meat products from other sources such as environmental contamination can also contribute to the formation of nitrosamines. The European Food Safety Authority’s experts concluded that these levels of nitrosamines might give rise to potential health concerns but that more research was needed to address uncertainties and knowledge gaps in this complex area ¹⁰.

Based on the available evidence, the European Food Safety Authority’s experts concluded that existing safe levels for nitrites and nitrates added to meat and other foods are sufficiently protective for consumers. Using more realistic data (i.e. actual concentration levels in food), the experts estimated that consumer exposure to nitrate solely from its use as a food additive was less than 5% of the overall exposure to nitrate in food, and did not exceed the acceptable daily intake (ADI) at 3.7 mg/kg body weight per day. For nitrites used as food additives, experts estimated exposure to be within safe levels for all population groups, except for a slight exceedance in children whose diet is high in foods containing these additives.

If all sources of dietary nitrate are considered (food additives, natural presence in food and environmental contaminants), the ADI may be exceeded for individuals of all age groups with medium to high exposure. Nitrite exposure from all dietary sources may exceed the ADI for infants, toddlers and children with average exposure, and for highly exposed individuals of all age groups.

To reduce uncertainties, the European Food Safety Authority panel made several recommendations, including:

• additional studies to measure the excretion of nitrate into human saliva, its conversion to nitrites, and the resulting methemoglobin formation;
• further studies on the levels of nitrosamines formed in different meat products based on known amounts of added nitrites/nitrates;
• large-scale epidemiological studies on nitrite, nitrate and nitrosamine intake and risk of certain cancer types.

What happens to nitrites/nitrates in the body?

In humans, nitrite and nitrate from food are rapidly absorbed by the body and, for the most part, excreted as nitrate. Some of the nitrate absorbed by the body is recirculated through salivary glands and part of it is converted by mouth bacteria into nitrite. Absorbed nitrite can oxidize hemoglobin to methemoglobin, an excess of which reduces the ability of red blood cells to bind and transport oxygen through the body. Nitrite in food (and nitrate converted to nitrite in the body) may also contribute to the formation of a group of compounds known as nitrosamines, some of which are carcinogenic.

Formation of nitrosamines

The formation of nitrosamines from nitrite within preserved food is possible, and the formation of N-nitroso compounds in nitrite-preserved meat and fish has been reviewed ¹¹. The main sources of N-nitroso compounds in the diet are nitrite preserved meat products ¹¹. Haorah et al. ¹² reported a mean concentration of 5.5 mol/kg N-nitroso compounds in frankfurters, but only 0.5 mol/kg N-nitroso compounds in fresh meat.

More than 80% of over one hundred N-nitroso compounds tested proved to be carcinogenic in animal experiments giving rise to tumors in many organs and also producing tumors transplacentally. N-nitroso compounds are carcinogenic in a wide range of animal species; most are mutagenic in test systems and some have been shown to be teratogenic to animals.

The possible health hazard from N-nitroso compounds is not confined to those present in the environment. Their formation, from a variety of precursors in the body of animals, has been demonstrated, and this may also occur in man.

A dose-response relationship has been shown to exist in different species of rodents for some carcinogenic N-nitroso compounds. As the dose is reduced, the tumor incidence decreases and the time for tumor induction increases and may exceed the life span of the animals.

Although there is no clinical or epidemiological evidence, it is highly probable that these compounds are also carcinogenic to man. However, present limitations concerning available dose-response data in animals and their interpretation, and inadequate knowledge of the biomechanism of cancer induction preclude a quantitative estimation of the carcinogenic risk to man that may be associated with different exposures to N-nitroso compounds.

A study was conducted in the USA to determine the dose of ascorbic acid required to inhibit N-nitrosoproline (N-nitroso compound) formation when 400 mg of nitrate were given 1 hour before a standard 400-calorie meal with 500 mg of L-proline. Volunteers ate their normal diets but restricted their intakes of nitrate, proline, N-nitrosoproline and ascorbic acid. N-Nitrosoproline and N-nitrosarcosine were determined in urine after 18 hour by methylation and then gas chromatography with thermal energy analysis. The mean yields of N-nitrosoproline were 11, 42, 33, 22 and 23 nmol in groups of 9–25 subjects taking proline alone, proline plus nitrate or proline plus nitrate plus 120, 240 or 480 mg of ascorbic acid, respectively. There was a significant trend to lower N-nitrosoproline yields as the dose of ascorbic acid increased. These results correspond to inhibition by ascorbic acid of 28%, 62% and 60% , respectively. Pairwise comparisons showed that each group taking ascorbic acid formed significantly less N-nitrosoproline than the group given only proline plus nitrate. The mean N-nitrosarcosine yields were 9 nmol when proline was taken alone and 17–24 nmol with proline plus nitrate plus ascorbic acid, with no trend to increasing individual values. It was concluded that 120 mg of ascorbic acid taken with a meal containing nitrate at 360 mg/day would significantly reduce nitrosamine formation in vivo ¹³.

Nitrate in food

Vegetables and fruit

Many data exist on the concentration of nitrate in vegetables but most of them are more than 20 years old. Reported values range from 30 to 6000 mg/kg ¹⁴. On the basis of published data, vegetables can be divided into three groups according to their nitrate content: low nitrate (< 100 mg/kg), medium nitrate (100–1000 mg/kg) and high nitrate (> 1000 mg/kg) ¹⁵. Leafy vegetables such as lettuce, spinach, celery leaf and beetroot leaf have the highest nitrate concentrations (above 1000 mg/kg). In the Republic of Korea ¹⁶, the concentrations of nitrate were found to be 4259 mg/kg in spinach while an average level of 1800 mg/kg was reported in radishes and Chinese cabbage. In Singapore ¹⁷, the concentrations of nitrate in salad, lettuce and spinach were 1360, 1470 and 4570 mg/kg, respectively. In Italy ¹⁸, the concentration of nitrate in leafy vegetables was reported to range between 80 and 6250 mg/kg, with a higher mean concentration in those obtained by organic farming both for green salad (1680 versus 3009 mg/kg) and chicory (3232 versus 4629 mg/kg). In France ¹⁹, the median concentration of nitrate in conventional and organic products was found to be 1591 and 1135 mg/kg, respectively, in spinach and 804 and 1221 mg/kg, respectively, in lettuce. In Poland, the concentration of nitrate in lettuce was found to be up to 3500 mg/kg ²⁰. In the United Kingdom between 1996 and 1998 ²¹, the average concentration of nitrate in lettuce produced in glasshouses was 2382 and 3124 mg/kg in the summer and winter, respectively, whereas that in lettuce produced outdoors was 1085 mg/kg. In the same study, the concentration of nitrate in spinach was 1900 mg/kg. In a more recent survey in the same country ²², the average concentration of nitrate in lettuce produced in glasshouses in summer was 2999 mg/kg (range, 676–4382 mg/kg; n = 18); in winter, the average was 3617 mg/kg (range, 1945–5720 mg/kg; n = 33). In lettuce produced outdoors, the concentrations were lower both in summer (average, 1140 mg/kg; range, 181–2656 mg/kg) and in winter (average, 1997 mg/kg; range, 810–3100 mg/kg); the average concentration of nitrate in spinach was 1815 mg/kg (range, 141–3909 mg/kg).

A second category of vegetables includes potatoes, cabbage and spring greens, which have concentrations that range between 100 and 1000 mg/kg. In Singapore ¹⁷, the concentrations of nitrate were 930, 340, 210, 150 and 140 mg/kg, respectively, in cabbage, green beans, aubergines, ladies finger [okra] and potatoes. In Poland, green beans contained up to 800 mg/kg ²⁰. In France, concentrations of nitrate in carrots were 113 and 394 mg/kg, respectively, for conventional and organic products ²³. In the same study, French beans were found to contain 711 and 561 mg/kg nitrate, respectively, in conventional and organic products.

In a third category, vegetables such as asparagus or onions and fresh fruit, including tomatoes, had the lowest concentrations (less than 100 mg/kg). The average concentration of nitrate in fresh fruit in the United Kingdom ²⁴ was found to be 27 mg/kg (range, 12–46 mg/kg). In Singapore ¹⁷, the concentrations of nitrate in tomatoes, asparagus, onions and mushrooms were found to be 60, 55, 35 and 15 mg/kg, respectively. In the Republic of Korea ¹⁶, the concentrations of nitrate in onions, soya bean sprouts and green peppers were found to range between 23 and 76 mg/kg. In France ²³, the concentration of nitrate in tomatoes was found to be 19 and 1 mg/kg in conventional and organic products, respectively. In Poland ²⁰, the concentration of nitrate in various fruit (currants, gooseberries, raspberries and cherries) ranged between 1.3 and 36 mg/kg, while a concentration of 58.7 mg/kg was found in strawberries.

Cereal grains and their products

The mean concentration of nitrate in bread and miscellaneous cereals obtained in total diet studies in the United Kingdom ²⁴ was 7.2–11 mg/kg (range, undetected–20 mg/kg). The mean nitrate contents reported by Walker ²⁵ and Gangolli et al. ²⁶ were between 0.5 and 16 mg/kg in cereals and cereal products worldwide. The variability of these results is easily explained by the various locations of production, seasons of sampling and type of products analyzed. The concentration of nitrate increased after baking; darker breads or breads that contained rye had slightly higher levels. In bread crumbs and pasta, the concentrations were reported by the same authors to range between 15 and 24 mg/kg.

Milk and dairy products

In the Total Diet Study in the United Kingdom, the average concentration of nitrate in dairy products was 27 mg/kg and that in milk was 3.9–5.3 mg/kg ²⁴. The concentration of nitrates in cows’ milk is generally below 5 mg/L ²⁶. In cheese without nitrate additives, the concentration of nitrates was found to be in the range of 1–8 mg/kg ²⁵.

Eggs

The average concentration of nitrates in eggs was estimated in the United Kingdom Total Diet Study ²⁴ to range between 4.4 and 5.4 mg/kg (range, undetected–12 mg/kg).

Cured meat

A survey of nitrate and nitrite preservative samples obtained from supermarkets and other retail outlets in England and Wales has recently been completed. A total of 200 samples, 40 each from Wales, northern England, South East England, South West England and the Midlands, were purchased and analyzed between December 1996 and February 1997. The sampling programme was designed to reflect recent changes in shopping patterns towards freshly sliced produce available from supermarket delicatessen counters or local butchers’ shops. Samples were transported to the laboratory and processed immediately ²⁴.

Fish

In the United Kingdom Total Diet Study ²⁴, the average concentration of nitrate in fish was estimated to be 11 mg/kg (range, 5–19 mg/kg).

The concentration of nitrate in fish was generally found to be below 2.5 mg/kg. It should be noted that, because nitrate and nitrite are authorized as food additives in certain countries, higher concentrations, ranging up to 380 mg/kg, were also reported ²⁵.

Beer

In the United Kingdom, the Food Standards Agency ²⁴ reported a mean concentration of nitrate in beers of 30 mg/kg (range, 10–100 mg/kg) in one survey. In a later survey, the mean concentration was found to be 16 mg/kg (range, 0.2–140 mg/kg) ²⁶.

Water

The natural occurrence of nitrate and nitrite in soil results from microbial oxidation of ammonia which is derived from organic nitrogenous material such as plant proteins, animals and animal excreta ²⁷. The background concentration of nitrate in sources of water in the USA is very low, and is generally less than 2 mg/L nitrate-N in groundwater and less than 0.6 mg/L in streams. Higher levels of nitrate are found in contaminated sources, and especially in groundwater ²⁸.

Intensive use of nitrogen fertilizers in crop culture is usually the main source of water contamination, but manure and slurries from concentrated animal feeding operations may be a significant source in some areas. Other anthropogenic sources are disposal of municipal effluents by sludge spreading on fields and deposition of airborne nitrogen compounds emitted from the combustion of fossil fuels by industry and automobiles ²⁸. Point sources include septic tanks, old landfills and leaking sewage systems ⁹.

Nitrite is a relatively unstable form of nitrogen that is rapidly converted to nitrate by bacteria; thus, the concentration of nitrite in environmental media such as water is usually very low, even when the concentration of nitrate is high ²⁹.

Public water supplies are rarely heavily contaminated by nitrate because they are usually more closely protected and water authorities are obliged to comply with certain water quality standards. Private wells, the quality of which is not usually regulated, are at greater risk of contamination, in particular shallow wells that  are located in agricultural areas. Nitrate is one of the major causes of eutrophication in surface water which leads to abundant growth of algae and aquatic plants. As a result, the concentration of nitrate in surface water is usually lower than that in groundwater ²⁹.

Despite a reduction in the use of commercial nitrogen fertilizers in Europe in the 1990s, no substantial reduction in the pollution of groundwater by nitrate has been observed in recent years ³⁰.

Nitrite in food

Vegetables

The average concentration of nitrite in vegetables is generally below 2 mg/kg. A mean concentration of 0.5 mg/kg was found in the United Kingdom Total Diet Study ²⁴. In Korean vegetables ¹⁶, an average concentration of 0.6 mg/kg was found.

Cereal grains and cereal products

The average concentration of nitrite in cereal products was reported to be 5 mg/kg in one study in the United Kingdom and 4 mg/kg in another study in the USA ²⁵. In the 1997 Total Diet Study in the United Kingdom ²⁴, the concentration of nitrite was found to range between undetected and 1.8 mg/kg.

Fish

The concentration of nitrite in fish was found to be below 0.6 mg/kg in several reports ²⁴.

Cured meat

‘Cured meat’ generally means meat that has been preserved using curing ingredients consisting of food-grade salt and sodium or potassium nitrite, and includes ham, bacon and sausages; it may also include dry-cured meat or meat cured by other processes that do not use nitrite. From a practical point of view, analytical surveys, and in particular the Total Diet Study, are not designed to determine nitrite content only and investigate preserved meat in general. It is therefore difficult to rule out the fact that some samples of meat analyzed may have been preserved or cured without the use of nitrites, which could lead to a ‘dilution’ of the reported average concentration. Nevertheless, such sampling is representative of the exposure of consumers to various preserved meats. In the European Prospective Investigation into Cancer and Nutrition (EPIC) cohort study, cured meat mostly meant nitrite-preserved meat, although some other meat preparations were included as cured meat ³¹. In the USA and western Europe, but not in China, processed meat is a more abundant commodity than processed fish and is the main source of dietary nitrite. It is therefore essential that the effects of cured meat be distinguished from those of fresh meat.

In the United Kingdom Total Diet Study, the mean concentration of nitrite in cured meat was 4.1 mg/kg (range, 1.5–8.4 mg/kg), while, in a more specific study conducted in the United Kingdom, the average residue level of nitrite was found to be 24 mg/kg (range, 0.2–170 mg/kg) ³². Such a result is probably related to
the inclusion in the Total Diet Study of samples of meat products that did not contain nitrate or nitrite as additives. A survey conducted in the USA ³³ of residual nitrite in 164 samples of cured meat in three trials showed average concentrations of 5–10 mg/kg (range, 0–48 mg/kg). Results reported in the 1970s showed average residual nitrite to be 52.5 mg/kg (range, 0–195 mg/kg).

In a survey from 1981, sausages (e.g. hot dogs) had a mean content of about 100 mg/kg nitrite and fried bacon and fried ham contained about 35 mg/kg nitrite ⁵. In a report that compiled 85 studies conducted between 1970 and 1991 in Canada and the USA of nitrite levels in cured meat, modelization of the results suggested some reduction in nitrite levels during the study period in most types of meat studied, except for frankfurters ³⁴.

Permissible levels in food

The maximum permitted level for nitrate in the Codex draft General Standard for Food Additives is 37 mg/kg in cheese and cheese products, 3650 mg/kg in processed meat, poultry and game, 360 mg/kg in comminuted [powdered] meat poultry and game products and 220 mg/kg in semi-preserved fish and fish products. The maximum permitted level for nitrite is 17 mg/kg in cheese and cheese products, 420 mg/kg in processed meat, poultry and game (whole pieces and cuts) and 130 mg in comminuted processed meat and in processed fish ³⁵.

Permissible levels of nitrate (used as a source of nitrite) in meat and poultry products used to be in the 250–500 ppm [mg/kg] range, but in recent years the trend has been to reduce levels of nitrate (and combined nitrate and nitrite) to inhibit the formation of nitrosamines. In the European Union, it has been suggested that input levels of nitrate for heat-treated meat products could be reduced to 150 ppm and residual levels to 100 ppm. In Japan, permissible nitrate input for meat products is 70 ppm, measured as residual nitrite; in the European Union, up to 50 ppm sodium or potassium nitrate (as sodium nitrate) can be used in foie gras; and in the USA, nitrate cannot be used to cure bacon. The content of nitrate is limited to 200 mg/L in raw milk for cheese in Japan and to 50 ppm in cheese in the European Union ³⁶, ³⁷, ³⁸.

Permissible residual levels of nitrite (usually expressed as sodium nitrite) in meat and poultry products generally range from 50 to 200 ppm (Australia/New  Zealand, European Union, Japan, USA). Permissible levels are typically 120–175 ppm for pork bacon (European Union, USA) and 70 ppm for whale bacon (Japan). Nitrite is also used to cure fish products; for example, in Japan, permissible residual levels are up to 50 ppm in fish ham and fish sausage and no more than 5 ppm in certain salted fish products; in the USA, levels up to 200 ppm are permitted in certain smoked fish ³⁸.

In some regions, additional limits are placed on baby foods. In the European Union, nitrate is limited to 200 ppm and, in the USA, no nitrate or nitrite can be added to food specifically produced for babies or young children ³⁶.

The addition of ascorbate as an antioxidant is also regulated in some countries. Regulations in the USA specify that, in addition to sodium nitrite or potassium nitrite, 550 ppm sodium ascorbate or sodium erythorbate should be added to prevent the formation of nitrosamines ³⁷. The maximal levels reported by the Directive 95/2/EC of the European Commission ¹ for most products, including pre-packed preparations of fresh minced meat, are ‘quantum satis’ [quantity required].

Sodium nitrite uses

Sodium nitrite has been used extensively in curing meat and meat products, particularly pork products such as ham, bacon and frankfurters; certain fish and poultry products are also cured with brines that contain sodium nitrite ³⁹. The process may include dry curing, immersion curing, or direct addition or injection of the curing ingredients. Curing mixtures are typically composed of salt (sodium chloride), sodium or potassium salts of nitrite and nitrate and seasonings. Sodium nitrite acts as a color fixative and inhibits the growth of bacteria, including Clostridium botulinum, which is the source of botulism toxin. Nitrite is a relatively strong reducing agent that has antibacterial
properties; however, the preservation of foodstuffs can be attributed to a large degree to the high concentration of salts (including nitrate) that are employed during the curing process. In addition, nitrate can act as a reservoir from which nitrite may be formed by microbiological reduction ⁴⁰.

In addition to the preservation of meats, curing is a process that develops and maintains a stable red color in cured and smoked meats, for which sodium or potassium nitrate or nitrite is responsible. When nitrate is used as the curing agent, the conversion (reduction) of nitrate to nitrite by bacteria in the meat or poultry is a necessary step in the development of the cured color. The amount of nitrate that is reduced to nitrite is dependent upon the number of nitrate-reducing bacteria and several environmental conditions such as temperature, moisture content, salt content and pH. Hence, the conversion rate and subsequent amount of nitrite that is formed is difficult to control. Similarly, the further reduction of nitrite to nitric oxide, which reacts with myoglobin (muscle pigment) to produce the cured color, is affected by the same environmental conditions. When nitrite is used as the curing agent, there is no need for the nitrate reduction step, and the development of the cured color is much more rapid. When the cured meat is heated, exposed to a more acid environment or left for a sufficient length of time under normal conditions, the nitric oxide–myoglobin is converted into a stable red pigment called nitrosohemochrome. The time required for a cured color to develop may be shortened with the use of cure accelerators, such as ascorbic acid and erythorbic acid or their derivatives, sodium ascorbate and sodium erythorbate. Cure accelerators speed up the chemical conversion of nitrous acid to nitric oxide, help to keep myoglobin in a reduced state and also serve as oxygen scavengers to prevent the fading of the color of cured meat in the presence of sunlight and oxygen ³⁹.

Other uses

Sodium nitrate is also used as a fertilizer, but only it accounted for only 0.1%, although it was still preferred for some specific crops and soil conditions.

Sodium nitrate is used in several industrial processes, in most of which it acts primarily as an oxidizing agent. A major use is in the manufacture of medium and high quality glass, such as optical and artistic glass, television and computer screens and fiberglass. In the manufacture of explosives, sodium nitrate is used mainly in blasting agents. Another large application is as an ingredient in the production of charcoal briquettes. Sodium nitrate is also used in the manufacture of enamels and porcelain as an oxidizing and fluxing agent, in formulations of heat-transfer salts for heat-treatment baths for alloys and metals, in rubber vulcanization and in petrochemical industries. Other uses of sodium nitrate include the treatment of water, the melting of ice, in adhesives, cleaning compounds and pyrotechnics, in the nitration of organic compounds, in certain types of pharmaceutical production, in the refining of some alloys, for the recovery of lead, in the production of uranium and in the leaching of copper ore ⁴⁰. Nitrate is used for the treatment of angina pectoris ⁴¹.

Is sodium nitrite bad for you?

Short‐term and subchronic toxicity studies in rats showed, overall, that nitrate intake of up to 5% in the diet (equivalent to 4,500 mg sodium nitrate/kg body weight per day) did not result in adverse effects in rats. At higher dose levels, animals showed signs of methemoglobinemia leading to the death of the animals.

In vitro studies on sodium and potassium nitrate in bacteria and mammalian cells did not provide evidence of a genotoxic potential. In mammals, no reliable indication of genotoxicity was obtained in mice and rats exposed to nitrate by the oral route, both in somatic and in germ cells. Although the database was limited, the European Food Safety Authority Panel concluded that the available experimental data indicated that nitrate salts do not raise concern for genotoxicity.

Chronic toxicity and carcinogenicity studies with sodium and potassium nitrate were available. In studies with mice, sodium nitrate did not show any difference in tumour incidences compared to controls. Four non‐standard studies in rats and pigs assessed haematological parameters or effects on thyroid and thyroid‐related hormones ⁴². Overall, the European Food Safety Authority Panel considered that nitrate did not affect adrenal and thyroid glands function in animals and it was not carcinogenic in animal studies.

No effects were observed in a reproductive/developmental toxicity screening study (OECD TG 422) in rats administered potassium nitrate by gavage at doses up to 1,500 mg/kg bw per day. No developmental toxicity was observed in mice, rats, hamsters or rabbits receiving doses up to 400, 1,980, 280 or 206 mg potassium nitrate/kg bw per day by gavage, respectively. In a reproductive toxicity study in mice given potassium nitrate in drinking water, effects were observed on sperm count and testicular enzymes at the highest dose tested, at which also sperm abnormalities were observed; the no‐observed‐adverse‐effect level (NOAEL) in this study was 122 mg potassium nitrate/kg body weight per day. Histopathological changes in testis, epididymis and other sex organs were reported in this study. A conclusion could not be reached since the duration of the dosing in males in this screening study was limited and the number of animals tested was low. Overall, the Panel noted that although some effects were observed in sperm analysis and reproductive organs in this limited study in mice, no indications of reproductive toxicity were observed at higher doses in a rat study conducted according to OECD guideline TG 422.

Human non‐cancer effects were observed in the thyroid in several studies, suggesting that nitrate exposure altered human thyroid gland function by competitively inhibiting thyroidal iodide uptake. A large cohort study (n = 21,977 women) showed that increasing intake of nitrate from dietary sources was associated with an increasing occurrence of hypothyroidism ⁴³. In addition, in several studies, an enlarged thyroid or even goiter was observed when the intake of nitrate via drinking water was high. Other studies, however, showed no effects. Overall, there was some evidence to relate exposure to nitrate with the development of enlarged thyroid, goiter and hypothyroidism. The exposure levels were in the same range as that within which methemoglobinemia was observed. The human studies on goiter and the single study on hypothyroidism were considered not sufficient for deriving reference points for a health‐based guidance value.

The methemoglobin formation reported in animal studies can also be observed in humans. This effect occurred both, upon acute exposure as well as after chronic exposure, to nitrate. The effect was a consequence of the endogenous production of nitrite from ingested nitrate.

Hence, in the absence of other adverse effects and the unavailability of the original 1958 study used by JECFA and the database currently available, the European Food Safety Authority Panel decided that the most relevant approach for assessing the toxicity of nitrate would be methemoglobinemia induced by nitrite formed from nitrate excreted in the saliva, once absorbed. The European Food Safety Authority Panel review the reported information on secretion estimates of nitrate into the mouth in humans and observed that most estimates available varied between 20% and 25% of the dose ⁴⁴. Additionally, the Panel noted that these studies were quite old and had limitations (it might be possible to obtain more accurate estimates nowadays). The European Food Safety Authority Panel therefore considered that adequate, well conducted modern studies might decrease the uncertainty associated with this estimate.

Studies of Cancer in Humans

Ingestion of nitrate and nitrite can result in the endogenous formation of N-nitroso compounds in the presence of nitrosatable precursors that are contained in meats, fish and some common drugs. Nitrate is ingested from dietary sources and drinking-water. Vegetables are usually the primary source when levels of nitrate in the drinking-water are below 50 mg/L, which is the regulatory limit in many countries. Many vegetables contain vitamin C and other compounds such as polyphenols that inhibit endogenous nitrosation. Whereas nitrate from both vegetables and drinking-water is reduced in the body to nitrite, sources from vegetables probably result in less endogenous formation of N-nitroso compounds because of the presence of inhibitors of nitrosation. For these reasons, the International Agency for Research on Cancer Working Group evaluated ingested nitrate from dietary sources and drinking-water separately.

Epidemiological studies that assessed the relationship between nitrate in the drinking water and cancer have been primarily ecological in design and focused on stomach cancer. Fewer case–control and cohort studies were available for other cancer sites. Ecological studies can provide important information on causal inference when exposure circumstances contrast greatly (between regions or population subgroups) and migration of populations is limited, particularly if there is almost homogenous exposure within each region and there are no serious potential confounders ⁴⁵.

However, inference from ecological studies of exposure to waterborne nitrate is more difficult because of the complexity of and intra-individual variation in endogenous nitrosation. Specific subgroups of a population that have higher exposure to nitrosation precursors (from nitrate in water and amines and amides in the diet) and lower exposure to inhibitors of nitrosation (e.g. dietary antioxidants) are probably at highest risk. Ecological studies of nitrate in drinking-water, therefore, are not liable to be highly informative unless levels of exposure are high and exposure to waterborne nitrate and other factors that affect nitrosation are homogeneous across the population groups. Therefore, the International Agency for Research on Cancer Working Group gave much greater weight to case–control and cohort studies in their evaluation.

Most case–control and cohort studies of exposure to nitrate in the drinking-water have been conducted in areas where levels of nitrate in drinking-water supplies are elevated due to the use of nitrogen fertilizers. Some of the highest levels of nitrate are found in shallow wells and surface water supplies which contain high levels in the spring due to run-off of excess nitrogen. Public water supplies are monitored routinely, and historical data on levels of nitrate are available; however, exposure to levels greater than the maximum contaminant limit in the USA of 10 mg/L (as nitrate-N) ⁴⁶ or the WHO drinking-water guideline of 50 mg/L as nitrate ⁴⁷ are rare. Private wells tend to contain higher levels of
nitrate than public supply wells because they are unregulated, are frequently shallower and more poorly constructed and are often located in close proximity to sources of contamination by nitrogen (crop fields, animal feed lots, septic tank systems). Since historical data from monitoring are available, most studies have focused on populations who use public water supplies and have excluded populations who use private wells with potentially higher exposures.

Occupational groups that may be at risk of ingesting nitrate were also considered. Workers in the manufacture of nitrate-based fertilizer can have high exposures to dusts that contain nitrate (> 10 mg/m3). One study ⁴⁸ among men whose jobs entailed exposure to different levels of nitrates in dust (maximum, 5 mg/m3) demonstrated that, at the end of a workday, salivary nitrate levels from highly exposed workers were approximately twice those of workers who had no exposure to nitrate in dust; however, dietary intake of nitrate was not controlled and the range of salivary levels of nitrate among highly exposed workers overlapped with those of unexposed men in several regions of England. Furthermore, a twofold variation in nitrate in saliva in non-occupationally exposed groups has been observed even under controlled conditions ⁴⁹. The International Agency for Research on Cancer Working Group considered that the evidence for exposure to nitrate via ingestion was lacking in these studies and could not be quantified; these studies were therefore not reviewed.

A few case–control studies have investigated exposure to nitrate from tap-water and have measured levels of nitrate and/or nitrite at the current residence or at the residence at the time of diagnosis of disease. Levels of nitrate and/or nitrite may change over time; therefore, considerable misclassification of exposure may occur if current levels of exposure are used to estimate past exposure. Variation in levels of nitrate across the distribution system of a public water supply could also introduce misclassification of exposure. In addition, nitrate and/or nitrite in water may be correlated with other environmental exposures such as agricultural pesticides that may be potentially relevant to some cancers.

Dietary intake of nitrite occurs primarily from the consumption of cured meats and fish, bakery goods and cereal products. Nitrite is also found as a contaminant of drinking-water but only in unusual circumstances. Several epidemiological studies evaluated the risk for specific cancers among subjects who had a higher intake of nitrite and a lower intake of vitamin C; this dietary pattern is liable to result in an increase in the endogenous formation of N-nitroso compounds. Studies of this design were considered by the International Agency for Research on Cancer Working Group to be particularly informative in the evaluation of human carcinogenicity.

Studies that estimated intake of nitrite from all dietary sources were reviewed, but these that only evaluated consumption of cured meat and risk for cancer were not reviewed specifically since they do not represent complete dietary nitrite intake. This is because many, but not all, cured meats contain nitrite and because other foods can also be important sources of nitrite. The International Agency for Research on Cancer Working Group also noted the results of some studies that estimated intake of nitrite and preformed N-nitroso compounds.

Several ecological studies in high- and low-risk areas for stomach and oesophageal cancers evaluated the potential for endogenous formation of N-nitroso compounds using the N-nitrosoproline (NPRO) test developed by Ohshima and Bartsch ⁵⁰. Some studies also measured urinary excretion of nitrate, levels of nitrate and nitrite in the saliva and excretion of other specific N-nitroso amino acids. Excretion of NPRO was generally higher in high-risk areas; however, not all of the differences were statistically significant ⁵¹. Urinary and salivary levels of nitrate reflect exposures to nitrate from both dietary sources and drinking-water; therefore, these studies were evaluated as a separate group. However, the International Agency for Research on Cancer Working Group did not give substantial weight to these studies in their evaluation because of the ecological study design and because recent excretion of nitrate or levels of nitrate in saliva may not reflect past exposures.

Nine (longitudinal) case–control studies on previous nitrite intake and various cancer types were reviewed. For oral and laryngeal cancer, no association was found with nitrite intake. One study conducted in the USA reported a positive association with esophageal cancer, with Odds Ratios of 1.0 (reference category), 1.2 and 1.6 for persons with a daily nitrite intake of < 1.1 mg, 1.1–1.6 mg and > 1.6 mg, respectively. The odds ratios and the trend across odds ratios were not statistically significant, however. The association between nitrite intake and esophageal cancer was stronger, and it was significant for persons with a history of canker sores. Another study in the USA, however, found no association between nitrite intake and esophageal cancer, nor with the subtypes adenocarcinoma and squamous-cell carcinoma; a positive association was found only with gastric cancer other than of the cardia. A positive association with gastric cancer was also reported in an Italian case–control study (average consumption, 2.4 mg/day), while no association was found in a French study (average consumption, 1.9 mg/day).

Odds ratios are used to compare the relative odds of the occurrence of the outcome of interest (e.g. disease or disorder), given exposure to the variable of interest (e.g. health characteristic, aspect of medical history). The odds ratio can also be used to determine whether a particular exposure is a risk factor for a particular outcome, and to compare the magnitude of various risk factors for that outcome.

• Odds ratios=1 Exposure does not affect odds of outcome
• Odds ratios>1 Exposure associated with higher odds of outcome
• Odds ratios<1 Exposure associated with lower odds of outcome

An association of borderline significance was found between nitrite intake and urinary bladder cancer in men but not women of Japanese descent, nor in whites of either sex, in Hawaii, USA. Although a positive association was reported from a study in the USA between brain tumours in children and their mothers’ consumption of processed meat, no association was found with nitrite intake during gestation or in childhood in a recent case–control study from Israel. One study on nasopharyngeal cancer among Taiwanese reported no association with nitrite intake in adulthood (as reported by cases and controls), but a positive association was found with childhood nitrite intake as recalled by the mothers of the cases and controls. The validity of recall of remote dietary intake is questionable, however.

Two prospective cohort studies have been conducted on nitrite intake and cancer risk. A cohort study from the Netherlands, with 6 years of follow-up, on dietary nitrite and gastric cancer risk reported relative risks of 1.0 (reference category), 1.2, 1.2, 0.9 and 1.4 for increasing mean quintiles of nitrite intake of 0.01, 0.04, 0.09, 0.16 and 0.35 mg/day, respectively. Neither the relative risks nor the trend was significant. A Finnish cohort study, with 24 years of follow-up, reported no association with the incidence of stomach, colorectal, or head-and-neck tumors. The average nitrite intake by this cohort was reported to be 5.3 mg/day.

Relative risk or risk ratio is the ratio of the probability of an outcome in an exposed group to the probability of an outcome in an unexposed group. Relative risk is used in the statistical analysis of the data of experimental, cohort and cross-sectional studies, to estimate the strength of the association between treatments or risk factors, and outcome. Relative risk is used to compare the risk of an adverse outcome when receiving a medical treatment versus no treatment (or placebo), or when exposed to an environmental risk factor versus not exposed.

Assuming the causal effect between the exposure and the outcome, values of relative risk can be interpreted as follows:

• Relative risk = 1 means that exposure does not affect the outcome;
• Relative risk < 1 means that the risk of the outcome is decreased by the exposure;
• Relative risk > 1 means that the risk of the outcome is increased by the exposure.

Thus, some studies indicated increased risks for esophageal and gastric cancer; however, other studies – particularly prospective cohort studies – revealed no such association. The results for brain tumors in children and for urinary bladder cancer in adults were equivocal. Wide variation between the studied populations in the recorded intake of nitrite was noted. In none of these studies was a possible interaction between nitrite and nitrosatable amines evaluated in respect of cancer risk. The results of these studies and those of the epidemiological studies considered by the United Nations (FAO)/World Health Organization (WHO) Expert Committee on Food Additives (JECFA) at its forty-fourth meeting do not provide evidence that nitrite is carcinogenic to humans. In addition, studies on nitrate intake and cancer risk (of relevance because of the conversion of nitrate to nitrite) also did not provide evidence of a positive association ⁵².

European Food Safety Authority Panel Opinion

In epidemiological studies, the summary evidence for an association between nitrate exposure and each type of human cancer was categorized by the European Food Safety Authority as: (i) there was no evidence for an association, if studies indicate no association with a specific cancer; (ii) there was insufficient evidence, to (unequivocally) link to a cancer (e.g. few studies, contradictory results, etc.); (iii) there was some evidence for an association with a specific cancer (e.g. inconsistent results between cohort studies and case–control studies); (iv) there was evidence, for an association with a specific cancer (e.g. consistent results from cohort studies and case–control studies).

The European Food Safety Authority Panel concluded that there was no evidence for a positive association between: ingested nitrate and esophageal cancer and its subtypes esophageal squamous cell carcinomas and esophageal adenocarcinoma; ingested nitrate and gastric cancer or its subtypes gastric cardia adenocarcinoma and gastric non‐cardia adenocarcinoma dietary nitrate and colorectal cancer or colon or rectum cancer; ingested nitrate and pancreatic cancer; ingested nitrate and lung cancer; dietary nitrate and non‐Hodgkin lymphoma (NHL); ingested nitrate and breast cancer; ingested nitrate and renal cell cancer; and ingested nitrate and adult glioma or childhood brain tumors ⁵³.

There was insufficient evidence for a positive association between: nitrate from processed meat and colorectal cancer or its subtypes; drinking water nitrate and colorectal cancer or its subtypes; drinking water nitrate and non‐Hodgkin lymphoma (NHL); ingested nitrate and leukemia; ingested nitrate and ovarian cancer; ingested nitrate and bladder cancer; ingested nitrate and prostate cancer; and ingested nitrate and thyroid cancer ⁵³.

There were insufficient data to draw conclusions on: ingested nitrate and head and neck cancer and ingested nitrate and liver cancer ⁵³.

Sodium nitrite side effects

Nitrate, via reduction to nitrite as noted above, causes methemoglobinemia, especially in infants. Methemoglobinemia is a blood disorder in which the body cannot reuse hemoglobin because it is damaged. Hemoglobin is the oxygen-carrying molecule found in red blood cells. In some cases of methemoglobinemia, the hemoglobin is unable to carry enough oxygen to body tissues. High concentrations of methaemoglobin are associated with hypotension, as a result of the vasodilatory effects of nitrite. Epidemiological evidence suggests a possible association between nitrate in the drinking-water and spontaneous abortions, intrauterine growth restrictions, birth defects, childhood onset of diabetes mellitus, thyroid hypertrophy, hypertension and recurrent diseases (respiratory tract infection, diarrhoea, stomatitis) in children. No teratogenic effects were observed in tests with nitrate and nitrite.

Symptoms of sodium nitrite poisoning usually occur within minutes of ingestion, are severe and can be fatal. They include:

• shortness of breath
• racing heart
• tiredness
• blue skin
• vomiting
• loss of consciousness.

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