Exceedance of the RfD does not imply that an adverse health effect would necessarily occur. As the amount and frequency of exposures exceeding the RfD increase, the probability of adverse health effects also increases. The highest levels of airborne formaldehyde have been detected in indoor air, where it is released from various consumer products. One survey reported formaldehyde levels ranging from 0.10 to 3.68 ppm in homes. The major sources appear to be power plants, manufacturing facilities, incinerators, and automobile exhaust emissions.
Smoking is another important source of formaldehyde.
Formaldehyde may also be present in food, either naturally or as a result of contamination. However, these measurements only appear to be useful when exposure to relatively large amounts of formaldehyde has occurred. The major toxic effects caused by acute formaldehyde exposure via inhalation are eye, nose, and throat irritation and effects on the nasal cavity. Other effects seen from exposure to high levels of formaldehyde in humans are coughing, wheezing, chest pains, and bronchitis. Ingestion exposure to formaldehyde in humans has resulted in corrosion of the gastrointestinal tract and inflammation and ulceration of the mouth, esophagus, and stomach.