Formic acid occurs in a variety of plants and fruits, mammalian tissues, and insect venoms. It is used industrially in preparing a variety of drugs, dyes, and chemicals; as a decalcifier; and in leather tanning. Formic acid also is an environmental contaminant of air and water and has been identified as the toxic intermediate (formate) in methanol poisoning. Two- and 13-week toxicity studies of formic acid were conducted in male and female F344/N rats and B6C3F1 mice by whole body inhalation exposure to formic acid vapors. In addition, in vitro genetic toxicity studies were performed with Salmonella typhimurium, with or without metabolic activation. Formic acid was not mutagenic in this assay.
In 2-week studies, groups of 5 F344/N rats and 5 B6C3F1 mice of each sex were exposed to formic acid for 6 hours a day, 5 days a week, at concentrations of 0, 31, 62.5, 125, 250, or 500 ppm. Deaths occurred in animals exposed to 500 ppm (rats and mice) and 250 ppm (1 female mouse). Microscopic lesions in the respiratory and olfactory epithelia occurred in rats and mice exposed to 62.5 ppm and higher concentrations, with the severity related to the exposure concentration. The lesions consisted of squamous metaplasia, necrosis, and inflammation. Exposures had minimal or no effects on coagulation times, blood pH and electrolytes, or on concentrations and activities of urine analytes in rats during the 2-week studies.
In 13-week studies, groups of 10 animals of each species and sex were exposed to formic acid at concentrations of 0, 8, 16, 32, 64, and 128 ppm for 6 hours a day, 5 days a week. Two mice, 1 male and 1 female, died in the 128 ppm groups. Body weight gains were significantly decreased in mice exposed to 64 and 128 ppm formic acid. Microscopic changes in rats and mice ranged from minimal to mild in severity and generally were limited to animals in the 128 ppm groups. Lesions related to exposure to formic acid consisted of squamous metaplasia and degeneration of the respiratory and olfactory epithelia, respectively. Hematologic and serum biochemical changes at interim and terminal time points were minimal to mild and, generally, were consistent with hemoconcentration.
Overall, the effects of formic acid were consistent with those of irritant chemicals administered by inhalation exposure. The no-observed-adverse-effect level (NOAEL) for respiratory injury was 32 ppm in rats and mice. There was no significant evidence of systemic toxicity in these studies.