Students at the University of California, Riverside set out to test how roof tiles coated with TiO2 affect nitrogen oxides in a chamber. They found that a thick coating could remove 97% of the target gasses. However, another tile, with just one twelfth the catalyst, removed 88%, suggesting a much more cost effective option. They estimate just $5 will cover the cost of coating for a typical new roof.

The students  have calculated that an average sized American home roof could, with the thinner coating, remove 21 grams of nitrogen oxide a day. This is equivalent to the amount of pollution a car releases in a year.