Researching on indoor air pollution recently, I discovered this study from 2005 on the "half-life" of formaldehyde in homes, based on the age of the house and the types of materials used. It makes for thought-provoking reading. Here's the abstract and a link:
"Decay is the decrease in formaldehyde concentrations in homes or the decrease in emissions from formaldehyde containing products over time. The decrease in formaldehyde concentrations over time (decay) in home studies is typically determined by associating formaldehyde concentrations by home age. The average half life in such studies is highly variable, varying from about 1 year to more than 20 years, depending on the nature of the home population under study and other factors.