A study in Ankara was performed to examine any association between formaldehyde levels and housing characteristics. This involved collection of 309 air samples which were analysed using a colorimetric method. Formaldehyde concentrations ranged from 2.34 mu g(.)m(-3) to 866.22 mu g(.)m(-3) (median, 71.48 mu g(.)m(-3)). Formaldehyde levels were elevated in homes inhabited by at least one smoker and were correlated with indoor and outdoor temperature, age of house and density of plywood furniture. Sufficient sample numbers were collected to allow the data to be statistically divided into five daytime periods. These data suggest that the formaldehyde concentrations were generally slightly higher in the afternoon. A simple mathematical model involving smoking activity has been developed for estimating the formaldehyde decay rates for two rooms in one of the typical background homes. When the windows and doors were closed, the formaldehyde concentrations in indoor air decreased within 10h to background concentrations, although this change depended on the room volume and the natural air exchange rate.