Urinary formic acid as an indicator of occupational exposure to formic acid and methanol.

A sampling strategy was developed to detect personal exposure to methanol and formic acid vapors. Formic acid is the metabolic end product of methanol, and part of inhaled formic acid is excreted directly in urine, so that urinary formic acid would reveal exposure to both agents. A linear relationship to inhaled vapors, however, could be shown only if urinary sampling were delayed until 16 hr (next morning) after exposure. Exposure to methanol vapor at the current Finnish hygienic limit level (200 ppm) produced 80 mg formic acid/g creatinine; exposure to formic acid at the hygienic limit (5 ppm) caused 90 mg/g creatinine. The similarity of these figures may indicate a common toxicological foundation of these empirically set values.