Asthma is a complex pulmonary inflammatory disease, which is characterized by airway hyper-responsiveness, airflow obstruction, and airway inflammation. Exposure to a number of chemicals including formaldehyde (FA) can lead to asthma. This study aimed to explore the underlying role of FA exposure in occupational asthma, especially when it is combined with allergen exposure. Balb/c mice were randomly divided into six groups (n = 6/group): (1) saline control; (2) ovalbumin (OVA)-immunized (OVA(imm)) only; (3) 0.5 mg FA/m(3) exposure; (4) OVA(imm) + 0.5 mg FA/m(3); (5) 3.0 mg FA/m(3) FA exposure; and, (6) OVA(imm) + 3.0 mg FA/m(3). These low and high exposure FA levels were adopted from current (0.5 mg/m(3)) and original (3.0 mg/m(3)) Chinese Occupational Threshold Limit Values. Experiments were conducted after 3 week of combined exposure and a 1-week challenge with aerosolized OVA. Airway hyper-responsiveness, pulmonary tissue damage, eosinophil infiltration, and increased interleukin (IL)-4 and IL-6 levels in lung tissues were found in the OVA + 3.0 mg FA/m(3) hosts as compared to values seen in the OVA-immunized only mice. The results here suggest to us that FA exposure can induce and aggravate asthma in Balb/c mice when it is combined with OVA immunization.