7.2 Toxicity
7.2.1 Human data
7.2.1.1 Adults
The lethal oral dose of potassium nitrate for an adult has been estimated to be between 4 and 30 g (about 40 to 300 mg NO3- kg). It has been reported that adults have tolerated large doses of nitrate as sodium and ammonium salt (> 100 mg NO3-/kg) in some cases repeated for several days for medical or experimental purposes with only minor effects in some subjects (light methaemoglobinemia, diarrhoea, vomiting). Death and severe effects of nitrate ingestion are generally associated with doses above 10 g NO3-.
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Human data
Severe methaemoglobinemia exceeding 70% may be associated with fatal outcome.
Methaemoglobin levels correlate well with symptoms in most cases (Hall et al., 1986):
0-3% Normal level
3-10% No clinical symptoms
10-15% None or slate grey cutaneous coloration "chocolate brown" blood
15-20% Generalized blue-grey cyanosis, usually asymptomatic
20-45% Headache, fatigue, dizziness, exercise intolerance, syncope
45-55% Increasing CNS depression
55-65% Coma, seizures, cardiac failure, cardiac arrhythmias, metabolic acidosis
> 65% High incidence of mortality
2.1 Main risks and target organs
The major acute toxic effect of nitrate and nitrite [poisoning] is methaemoglobinaemia. Blood is the target organ. Methaemoglobin reduces the oxygen-carrying capacity of the blood and in addition, it shifts the oxyhaemoglobin dissociation curve to the left interfering with the unloading of oxygen. Hypotension and collapse may also occur. The principal concern with exposure to nitrate is its biological reduction to reactive and toxic nitrite. Nitrate itself is rather harmless.
4.2 High risk circumstances of poisoning
Accidental exposure: Accidental addition of nitrates/nitrites to food in mistake for common salt has resulted in poisoning.
Intentional exposure: Employees with access to nitrites at work, e.g. laboratory personnel, have occasionally attempted suicide through ingestion of nitrite.
Medical exposure: Mashed carrot widely used to treat infant diarrhoea has occasionally resulted in intake of toxic amounts of nitrate (ECETOC, 1988).
Sodium nitrite given intravenously is traditionally used as an antidote in cyanide poisoning (see Section 5.5).
Other types of exposure: The major concern of nitrate and nitrite is associated with intake of food and water. Drinking water contains variable amounts of nitrate. The statutory limits vary slightly from country to country, but is usually either maximum 10 mg NO3--N/L(= 44.3 mg NO3-/L) in the USA or maximum 50 mg NO3-/L in the EU.
Plants contain nitrate as a normal cell constituent and vegetables are usually the main dietary source of nitrate. Normal daily intake varies with dietary customs, 50 to 150 mg NO3-/day seems typical for a western diet. Vegetarians can exceed this, with daily intakes of over 300 mg NO3- (Walker, 1990).
Dietary exposure to nitrites is normally very low, commonly <2 mg NO2-/day and usually <5 mg/day per capita (<0.1 mg/kg/day) (Walker, 1990). Exceptionally, higher levels may result from microbial reduction of nitrates in hygienically poor quality well water or in foods rich in nitrates stored under inappropriate conditions. Bacterial reduction of nitrate secreted in the saliva and gastric juice is usually the source of nitrite (Eisenbrand et al., 1980; Mueller et al., 1986). NAS (1981) estimated that approximately 3.5 mg NO2- is formed per day in an average adult in the USA. This process is dependent upon several factors (e.g. nitrate intake) and varies substantially between individuals.
6.2 Distribution by route of exposure
Regardless of route of exposure, nitrate and nitrite are rapidly transferred into the blood. Nitrite is gradually oxidized to nitrate which is readily distributed into mostly body fluids (urine, saliva, gastric juice, sweat, ileostomy fluid). Distribution of nitrate into plasma, erythrocytes, saliva and urine following an oral dose of sodium nitrate has been demonstrated by Cortas & Wakid (1991).
6.3 Biological half-life by route of exposure
Wagner et al. (1983) showed the half-life in the body for an oral dose of nitrate to be approximately 5 hours. As blood absorption depends on food matrix (see Section 6.1) and route of exposure, and as larger doses may increase the urinary excretion rate, the biological half-life for both nitrate and nitrite should be expected to be 3 to 8 hours. Nitrate does not accumulate in the body.
6.4 Metabolism
Where bacteria are present and the environment can be anaerobic, nitrate can be reduced to nitrite. The main site for this reaction is mouth and stomach, but nitrite formation in the lower intestine and in the bladder (urinary infection) may also be of some toxicological importance.
Nitrite may be further reduced to nitrogen by bacteria under some conditions. In blood, nitrite transforms haemoglobin to methaemoglobin and is simultaneously oxidized to nitrate. Normally methaemoglobin gradually reverts to haemoglobin through enzymatic reactions.
Nitrite has vasodilating properties, probably through transformation into nitric oxide (NO) or a NO-containing molecule acting as a signal factor for smooth muscle relaxation.
Nitrite easily transforms into a nitrosating agent in an acidic environment and can react with a variety of compounds, e.g. ascorbic acid, amines, amides.
Nitrosation can also be mediated by bacteria, e.g. in the stomach. Some reaction products are carcinogenic (e.g. most nitrosoamines and amides.
7.2 Toxicity
7.2.1 Human data
7.2.1.1 Adults
The lethal oral dose of potassium nitrate for an adult has been estimated to be between 4 and 30 [GRAMS] (about 40 to 300 mg NO3- kg).
It has been reported that adults have tolerated large doses of nitrate as sodium and ammonium salt (> 100 mg NO3-/kg) in some cases repeated for several days for medical or experimental purposes with only minor effects in some subjects (light methaemoglobinemia, diarrhoea, vomiting). Death and severe effects of nitrate ingestion are generally associated with doses above 10 g NO3-.
7.3 Carcinogenicity
There is no evidence that nitrate or nitrite as such cause cancer in animals (ECETOC, 1988). However, a causative connection between nitrate/nitrite and cancer through the formation of N-nitroso compounds is suspected.
The role of nitrate and nitrite in the etiology of cancer in humans, especially gastric cancer, is addressed in numerous studies which are reviewed and discussed by Walker (1990), Forman et al. (1989), ECETOC (1988), IARC (1987) and WHO (1985). Included are also epidemiological studies seeking to find correlation between frequency of cancer and nitrate intake with food and water. Evidence from these sources does not support the hypothesis of a straightforward cause and effect association between nitrate exposure and cancer risk (Forman, 1989).
7.6 Interactions
Methaemoglobinemia can also result from several other chemical compounds; e.g. Acetanilide, o-Aminophenol, p-Aminophenol, Aniline, Dimethylaniline, Hydroxylamine, p-Nitroaniline, Nitrobenzene, Nitro-glycerine and Amylnitrite (Clayton & Clayton, 1981). Cases have also been reported due to the use and overdose of some medicines, e.g. benzocaine, dapsone. There should thus be potential for synergism between methaemoglobin-forming substances and nitrite, but we are not aware of any studies on this topic.
9.1.1 Ingestion
Ingestion is the major route of exposure. The first symptoms may appear within 10 to 45 minutes. Methaemoglobinaemia is the principal and constant feature of nitrate/nitrite poisoning.
Clinical symptoms may include: nausea, vomiting, abdominal pain, headache, dizziness, fall in blood pressure, tachycardia, collapse, bluish-grey cyanosis, hyperventilation, stupor, convulsions, coma and death.
9.3 Course, prognosis, cause of death
In mild cases, gastrointestinal symptoms and asymptomatic cyanosis dominate the clinical presentation. In severe cases coma and death can occur in the first hour due to hypoxia (severe methaemoglobinaemia) and circulatory collapse. In case of parenteral administration the onset of methaemoglobinaemia is immediate. Prognosis is usually good if adequate treatment is provided.
Death due to nitrates and nitrites have resulted from large suicidal ingestions, ingestion of contaminated food, industrial accidents and ingestion of contaminated well water in neonates (Harris et al., 1979; Gosselin et al., 1984; Johnson et al., 1987; Donovan, 1990).
9.4 Systematic description of clinical effects
9.4.1 Cardiovascular
Acute: Nitrite produces relaxation of smooth muscle, especially in veins, but also in coronary and peripheral arteries. Venous pooling in the lower extremities leads to a decreased cardiac preload and output inducing hypotension and thus ischemia of vital organs. Hypotension and syncope induced by large doses of nitrite is due initially to the pooling of blood in dilated post-arteriolar vessels, notably venules and even large veins. This vasodilation is not blocked by atropine or by any recognized drug. Reflex tachycardia is the rule but a vasovagal reflex may induce transient bradycardia just before complete collapse (Gosselin et al., 1984; Donovan, 1990). This collapse can occur from marked vasodilation, decreased cardiac output and vital organ anoxia. Arythmias have been reported (Gowans, 1990) Electrographic changes of hyperkalemia (peaked T waves) have been reported by Sporer and Mayer (1991) in a 37-year-old man who had ingested saltpeter (potassium nitrate).
9.4.3 Neurological
9.4.3.1 Central Nervous System (CNS)
Headache, dizziness, restlessness, agitation and confusion are common in moderate poisoning. In severe cases, stupor, convulsions and coma can occur as a result of cerebral anoxia.
9.4.10 Haematological
Acute: Methaemoglobinaemia is the principal and constant feature of acute nitrate and nitrite [poisoning]. Methaemoglobin is haemoglobin in which the iron has been oxidized to the ferric state, Fe3+, rendering it incapable of oxygen transport. Methaemoglobin exerts its toxicity in two ways: (a) it reduces the oxygen-carrying capacity of the blood; (b) in addition, it shifts the oxyhaemoglobin dissociation curve to the left, interfering with the unloading of oxygen (Donovan, 1990; Goldfrank, 1990).
In mild cases, slate grey cyanosis may be visible only in the lips and mucous membranes.
The appearance of cyanosis also depends on the total haemoglobin, oxygen saturation, skin pigmentation, and ******t lighting.
9.4.15 Special risks
Human red cells deficient in glucose-6-phosphate dehydrogenase are more sensitive to the methaemoglobin-generating activities of nitrite than normal red cells (Gosselin et al., 1984). Patients with congenital NADPH Methb Reductase deficiency are also particularly susceptible to nitrates/nitrites.
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