The presence of formaldehyde in biologically active concentrations in several occupational and environmental atmospheres has created concern about the hazards of exposure to these contaminated atmospheres. A review of the clinical and animal toxicologic data demonstrate both primary irritant and sensitizing actions of the compound. The minimal concentration of formaldehyde that can be detected by odor is similar to the level that produces minimal irritant effects on the eyes and in the pulmonary airway, thereby serving as a warning of exposure to the compound. The irritant effects are reversible when exposure is discontinued. Some persons show allergic type responses to concentrations of the compound that are well below the odor threshold. The very limited amount of toxicologic data concerned with effects from prolonged and chronic exposures suggest that the compound does not produce specific delayed types of toxicity. I discuss the factors that influence establishment of "safe" conditions or levels of exposure to formaldehyde.