Background: Domestic air filtration units have previously been shown to cause a dramatic fall in airborne pet allergen levels in homes with pets. Clinical trials of air filtration units, however, have failed to reveal a significant beneficial effect. Personal pet allergen exposure during air filtration unit use has never been measured. Objective: To determine the effect of air filtration on inhaled cat allergen exposure in homes with cats. Methods: Nasal air samplers were worn to measure personal cat allergen exposure. The study was carried out in five homes with cats on 4 separate days examining four experimental conditions (cat absent or present, air filtration off or on). The two operators collected four baseline samples and two 15-min samples/h over three consecutive hours. Cat allergen-bearing particles were detected by immunoblotting and allergen concentrations measured by amplified enzyme-linked immunosorbent assay (ELISA). Results: There was a significant reduction in the quantity of the inhaled Fel d 1 when the air cleaner was used with the cat in the room. Fel d 1 halo counts (detransformed means) were 29.3 at baseline, 11.8 after 1 h, 10.0 after 2 h and 14.1 after 3 h, with no change on control days (P = 1.00). With the cat elsewhere in the house, a marginal, but statistically significant reduction was observed only after 3 h with the use of air cleaner (Fel d 1 halo count: baseline 12.4; 3 h 5.5; P=0.01). Conclusions: The use of air filtration units appears to result in a much smaller reduction of inhaled cat allergen exposure than suggested by previous studies using standard air samplers. Cat removal remains the best advice to cat-allergic patients who experience symptoms upon exposure.