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National Research Council (US) Committee on Toxicology. Formaldehyde - An Assessment of Its Health Effects. Washington (DC): National Academies Press (US); 1980.

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Formaldehyde - An Assessment of Its Health Effects.

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EFFECTS ON ANIMALS

SHORT-TERM STUDIES

Metabolism

Formaldehyde is a normal metabolite in mammalian systems and, in small quantities, is rapidly metabolized (Akabane, 1970). The major route of biotransformation appears to be oxidation to formic acid followed by further oxidation to carbon dioxide and water (Buss et al., 1964; Williams, 1959). Administration of radiolabeled formaldehyde to rats by the oral or intraperitoneal route resulted in 40% and 82%, respectively, of the label being found in respiratory carbon dioxide (Neely, 1964; Williams, 1959). The remaining isotope in the intraperitoneal study was in urine as methionine, serine, and an adduct formed from cysteine and formaldehyde (Neely, 1964).

Numerous enzymes capable of catalyzing the reaction of formaldehyde to formic acid have been identified in liver preparations and erythrocytes (Tephly et al. 1974; Utolia and Koivusalo, 1974). Williams (1959) characterized formaldehyde as a compound that reacts rapidly with amino acids, histones, and proteins to form both reversible methylol adducts and stable methylene bridges.

Mortality

Reported LD50 values of formaldehyde for the rat after oral administration ranged from 550 to 800 mg/kg (Tsuchiya et al., 1975; Smyth et al., 1941). The LC50 values for rats at 0.5 and 4 h were 820 and 482 ppm, respectively (Skog, 1950; Nagornyi et al., 1979). Pulmonary edema was the predominant pathologic change at these concentrations. Similar results were obtained in mice and cats (Nagornyi et al., 1979; Iwanoff, 1911).

Effects on the Eye

Formaldehyde is a severe eye irritant. Application of a drop of formalin to rabbit eyes caused edema of the cornea and conjunctiva and iritis, graded 8 on a scale of 1–10 (Carpenter and Smyth, 1946). Exposure of rabbits and guinea pigs to airborne formaldehyde at 40–70 ppm for 10 d produced some lacrimation, but no corneal injury (Grant, 1974).

Potts et al. (1955) injected formaldehyde intravenously (0.9 g/kg) in monkeys over several hours and observed an immediate change in the electroretinogram, but no blindness.

Effects on the Skin

Formaldehyde can cause skin irritation and is a potent allergen. Mild to moderate irritation developed when formaldehyde was applied to guinea pig skin in concentrations of 0.1–20% (Colburn, 1970). Guinea pigs are readily sensitized with intradermal injections (Draize method), topical occluded applications (Buehler method) and open epicutaneous tests (OET method) (Klecak, 1977; Magnusson and Kligman, 1977; Marzulli and Maibach, 1977). With open applications, a 3% solution of formaldehyde sensitized guinea pigs, whereas a–1% solution did not (Maibach, 1978). Preexisting sensitivity was elicited with concentrations down to 3% by open challenge, and 1% or less by closed-patch challenge. In another study, nine guinea pigs were administered formaldehyde intradermally or topically at 0.1–1% over a 2–wk period (Colburn, 1970). After a 2–wk rest period, the animals were challenged with formaldehyde at 0.01–5% and 2 d later at 0.02%; five became sensitized.

Effects on the Respiratory Tract

Formaldehyde is extremely soluble in mucous membranes of the respiratory tract. Egle (1972) concluded that retention is nearly 100% in dogs, regardless of ventilation rate, tidal volume, region of the respiratory tract exposed, or formaldehyde concentration.

Exposure for 10 min to 3.1 ppm produced a 50% decrease in respiratory rate (RD50) of mice (Kane and Alarie, 1977). Utilizing a tracheal cannula to deliver formaldehyde, these authors showed that the upper respiratory tract was the site of reactions that provoked the decrease in respiratory rate. Repeated exposures at 1.0 and 3.1 ppm, 3 h/d for 4 d revealed that the maximum percent decrease in respiratory rate was reached after about 4–8 min, and increased each day. After the plateau was reached, there was a reduction in the percentage decrease during the rest of the exposure. There was complete recovery between the daily exposures.

Concentrations of formaldehyde at 0.3–50 ppm significantly increased airway resistance and decreased lung compliance in guinea pigs after 1 h of exposure (Amdur, 1960). The magnitude of the effects were dose-dependent over the range of concentrations tested. These effects were reversible within 1 h after exposure at concentrations of 0.3–11 ppm. No effect was observed in guinea pigs exposed at 0.05 ppm for 1 h. Tracheal cannulation resulted in a greater increase in airway resistance, and the combination of formaldehyde and submicron particle size sodium chloride aerosol at 3–30 mg/m3 increased resistance even further. Other research has shown formaldehyde to depress ciliary activity within 10 min when tracheal preparations were exposed at concentrations of 20–100 ppm (Cralley, 1942; Dalhamn and Rosengren, 1971).

Effects on the Nervous System

Kulle and Cooper (1975) reported that a 1 h exposure to formaldehyde at 0.5–2.5 ppm decreased rat nasopalatine nerve response to amyl alcohol. A partial recovery of the neural response occurred when the nasal cavities were perfused with air for 1 h after the formaldehyde exposure. Bonashevskaya (1973) exposed rats at 0.83 and 2.5 ppm for 3 mo. Histologic and histochemical changes were observed in the neurons and dendrite receptor synaptic apparatus in the cerebral amygdaloid complex. No histologic changes were observed in the central nervous system of monkeys injected with formaldehyde intravenously (0.9 g/kg) over several hours (Potts et al., 1955).

PROLONGED STUDIES

A 90–d study was conducted with formaldehyde administered in the drinking water of rats on a weight/volume basis at 50, 100, and 150 mg/kg body weight/d (Monsanto, 1973a) or mixed in the diet so that dogs received 50, 75, and 100 mg/kg body weight/d (Monsanto, 1973b). There were no significant effects on hematologic (hematocrit, hemoglobin, and total and differential leukocyte counts) and biochemical (blood sugar, blood urea nitrogen, alkaline phosphatase, and serum glutamic oxaloacetic transaminase) parameters, or in several organs examined histologically. The highest dose administered to each species produced a decrease in weight gain.

Groups of 25 male rats were continuously exposed to formaldehyde by inhalation at 1.6, 4.6, or 8 ppm for up to 3 mo (Dubreuil et al., 1976). The only effect observed at the low dose was a yellowing of fur. The intermediate-dose group also showed decreased body weight. The group exposed at 8 ppm for 60 d showed eye and upper respiratory irritation, decreased body weight gain, and decreased liver weight. In another inhalation study, rats, guinea pigs, rabbits, monkeys, and dogs were exposed continuously at 3.8 ppm for 90 d (Coon et al., 1970). One of 15 exposed rats died, but no other signs of toxicity were observed. Various degrees of interstitial inflammation were seen in the lungs of all exposed animals. Focal chronic inflammation was also observed in the hearts and kidneys of the rats and guinea pigs. The authors were uncertain whether these inflammatory changes resulted from exposure to formaldehyde.

Groups of 60 mice were exposed to airborne formaldehyde at 41.5, 83, or 166 ppm 1 h/d, three times a week for up to 35 wk (Horton et al., 1963). Pathologic examination of the tracheal epithelium revealed basal cell hyperplasia, squamous cell metaplasia, and atypical metaplasia. Metaplasia extended into the major bronchi in the 41.5–ppm group after exposure at 125 ppm for an additional 29 wk. Exposure of mice at 166 ppm was terminated after 11 d, owing to intoxication and high death rate. In a noncontinuous inhalation study, mice and rats were exposed at 4, 12.7, and 39 ppm, 6 h/d, 5 d/wk (Battelle Columbus Laboratories, 1977a). No adverse effects were observed in the 4 ppm group exposed for 13 weeks. At 12.7 ppm for 13 weeks, decrease in body weight was observed; 2 of the 20 exposed rats showed evidence of nasal erosion. The 39–ppm exposure was terminated after 2 wk because of severe changes in nasal mucosa, including ulceration and necrosis.

In another study, 25 rats each were exposed continuously for 3 mo at 0.0098, 0.028, 0.82, and 2.4 ppm (Fel'dman and Bonashevskaya, 1971). The authors reported that at 2.4 ppm there was a significant decrease in cholinesterase activity, and at 0.82 and 2.4 ppm proliferation of lymphocytes and histiocytes in the lungs and some peribronchial and perivascular hyperemia. Exposure at the two lowest concentrations resulted in no significant findings.

CARCINOGENIC POTENTIAL

Mice exposed to formaldehyde at 83 ppm, for 1 h, 3 d/wk for 35 wk or at 41.5 ppm for 1 h, 3 d/wk for 35 wk and at 125 ppm for an additional 29 wk showed basal cell hyperplasia and squamous cell metaplasia in the tracheobronchial epithelium but no tumors (Horton et al., 1963). Hamsters exposed at 10 ppm for 5 h, 5 d/wk for their lifetime (average, 18 mo) showed increased cell proliferation and hyperplasia in the lungs (Nettsheim, 1976). This investigator also reported that weekly 5–h exposures at 50 ppm for lifetime (18 mo) produced squamous metaplasia, but no tumors.

Fischer 344 rats and B6C3F1 mice (120/sex/concentration) are being exposed to formaldehyde at 0, 2, 6, and 15 ppm for 6 h/d, 5 d/wk in a Chemical Industry Institute of Toxicology (CIIT) sponsored study at Battelle Columbus Laboratories (CIIT, 1979b). Preliminary results indicated that 15 ppm caused multifocal squamous cell metaplasia of nasal epithelium in 6 of 20 rats at both the 6– and the 12–mo sacrifice. Histologic examinations of 3 additional rats with enlarged noses after 15, 15, and 16 mo of exposure demonstrated squamous cell carcinomas in the nasomaxillary epithelium. A single squamous cell carcinoma of the skin was seen after 10 mo in the group of rats exposed at 6 ppm; this tumor was not of the same type observed at 15 ppm and did not invade the nasal epithelium. Additional preliminary results have shown the presence of nasal carcinomas in 8 of 40 rats exposed at 15 ppm and sacrificed at 18 mo and nasal carcinomas in 29 other rats exposed at 15 ppm that were moribund or died spontaneously between the sixteenth and eighteenth months (CIIT, 1980). Additional tumors have not been found in the group exposed at 6 ppm. A small adenomatous polyp was found in one of 40 rats exposed at 2 ppm and sacrificed at 18 mo. Epithelial dysplasia and squamous metaplasia of the turbinates were observed in rats in all three exposure groups, the magnitude of the effects being dose-related. No control rats or any mice have shown histopathologic changes or tumor development of the kinds found in exposed rats. This is the first study to implicate formaldehyde as a potential experimental carcinogen, but the significance of these preliminary findings can be evaluated only after completion of the study and analysis of the pathologic findings.

Injection-site sarcomas developed in 2 of 10 rats given weekly injections of 0.4% aqueous formaldehyde for 15 mo (Watanabe et al., 1954). Fibrosarcomas were observed in the liver and omentum in 2 other rats. These results are not meaningful, because of lack of controls and inappropriateness of the route of administration.

Rusch et al. ([1980]) exposed rats to HC1 at a mean concentration of 10.7 ppm and formaldehyde at 10.3 ppm for 6 h/d, 5 d/wk for 410 exposures over 618 d. Before dilution to the stated concentrations in the exposure chamber, the initial reaction mixture had average HCl and formaldehyde concentrations of 6,567 and 1,021 ppm, respectively; alkylating-agent activity of 1,813 ppb was also detected, possibly as a result of the interaction of HCl and formaldehyde in the gas phase. Alkylating-agent activity in the animal exposure chamber, as measured by chromatography, was 28 ppb. Preliminary results of histologic examinations on 56 exposed animals indicated a 14% incidence of squamous cell carcinoma of the nasal epithelium after 589 d. Tumors of this kind were not observed in controls. One of the alkylating agents identified in the chamber was bis(chloromethyl) ether (BCME), at a concentration of approximately 0.1 ppb. BCME is a potent carcinogen; esthesioneuroepitheliomas of the nose, squamous cell carcinomas of the lung and nasal turbinates, and adenocarcinomas of the lung and nasal cavity were produced in rats after exposure to BCME at 0.1 ppm 6 h/d, 5 d/wk for 10–100 exposures (Kuschner et al., 1975).

The carcinogenic potential of hexamethylenetetramine (HMT), which can decompose in an acid media to release formaldehyde and ammonia, has been examined (Della Porta et al., 1968). Mice and rats were given fresh solutions of HMT in drinking water every 24 h at 0.5–5% for 30–60 wk and at 1–5% for 2–104 wk, respectively. Mice were observed for up to 130 wk and rats for up to 3 yr. At 5% HMT, there was 50% mortality in the rats after 2 wk. No significant effects on growth or survival were observed in any of the other groups of rats or the mice. Histologic examination indicated that no effects were attributable to HMT. No carcinogenic activity was observed.

MUTAGENIC POTENTIAL

Numerous studies have been conducted to determine the mutagenicity of formaldehyde, and Auerbach et al. (1977) have reviewed the subject extensively. Formaldehyde has exhibited mutagenic activity in a wide variety of organisms, but the mechanism of formaldehyde mutagenesis has not been resolved. Formaldehyde may cause mutations by reacting directly with DNA; by forming mutagenic products on reaction with amino groups on simple amines, amino acids, nucleic acids, or proteins; or by oxidizing to peroxides that can react directly with DNA or indirectly by free-radical formation.

Mutagenic activity has been reported in E. coli (Bilimoria, 1975) and Pseudomonas fluorescens (Englesberg, 1952), but not in the Ames strains of Salmonella typhimurium (Koops and Butterworth, 1976). Weak mutagenic activity was observed when the fungi Neurospora crassa and Aspergillus nidulans were treated (Auerbach et al., 1977). The increase in mutagenic activity observed in these studies after treatment in the presence of catalase inhibitors suggested that peroxides were involved in the induction of mutations. Formaldehyde induced mitotic recombination in Saccharomyces cerevisiae (Chanet et al., 1975). The studies concerning formaldehyde mutagenesis in Drosophila have been reviewed by several authors (Auerbach et al., 1977; Rapoport, 1948; Solyanik et al., 1972). Mutations were induced in male larvae fed formaldehyde-containing food and in adults injected with aqueous solutions of formaldheyde. The exposure of adults or larvae to formaldehyde vapors has not produced mutations. In one of five species of grasshoppers, formaldehyde caused chromosomal damage (Manna and Parida, 1967). Germinating barley seeds soaked in formaldehyde solutions did not give evidence of mutation on maturation (Ehrenberg et al., 1956).

The mutagenic potential of formaldehyde in mammalian systems has not been thoroughly studied. An increase in mutation frequency was observed when formaldehyde was tested in the L5178Y mouse lymphoma assay (Gosser and Butterworth, 1977), according to the published procedure (Clive and Spector, 1975). A clear dose-response relationship was evident in only one of four experiments. No mutagenic activity was observed when formaldehyde was tested in the Chinese hamster ovary cell/HGPRT assay (Hsie et al., 1978). Likewise, no effect was observed in dominant lethal studies conducted with Swiss mice (Epstein et al., 1972).

Although formaldehyde exhibits mutagenic activity in a variety of microorganisms and in some insects, more work is necessary to ascertain the potential of this compound to cause mutations in germinal or somatic mammalian cells.

EMBRYOTOXIC/TERATOGENIC POTENTIAL

There were no adverse gonadotropic or reproductive effects in male rats administered formaldehyde at 0.1 ppm in drinking water or 0.4 ppm in the air for 6 mo (Guseva,1972). Pregnant dogs fed diets containing 125 or 375 ppm on days 4–56 of pregnancy showed no evidence of teratogenesis (Hurni and Ohder, 1973). There was no effect on the course of pregnancy and no malformations in the offspring when rats were exposed at 4 ppm, 4 h/d during days 1–19 of pregnancy (Sheveleva, 1971).

Gofmekler (1968) exposed female rats to airborne formaldehyde at 0.8 and 0.01 ppm for 10–15 d before placing them with males. All animals were then exposed for 6–10 d at the same concentrations of formaldehyde. Day of mating was not recorded and duration of gestational exposure was unknown. No gross abnormalities were observed in the offspring, but there was a 14–15% increase in duration of pregnancy, compared with controls. Although the surviving offspring of exposed mothers averaged slightly greater body weights than the offspring of controls, the lungs and livers of offspring of exposed mothers were smaller than those of controls. Histologically, the livers of the offspring of mothers exposed at 0.01 ppm were not different from those of controls. In the group from mothers exposed at 0.8 ppm, the livers showed increased extramedullary hematopoietic centers and epithelial proliferation in the common bile duct. Additional studies are needed before firm conclusions can be made about the teratogenic potential of airborne formaldehyde at low concentrations.

Dogs fed HMT at 600 and 1,250 ppm on days 4–56 of pregnancy did not show evidence of teratogenesis (Hurni and Ohder, 1973). Likewise, long-term feeding studies of rats given 0.16% HMT showed no effect on the reproductive capacity of rats (Natvig et al., 1971).

Copyright © National Academy of Sciences.
Bookshelf ID: NBK217651

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