The small study found that the more concentrated polycyclic aromatic hydrocarbons (which are from car exhaust) were in the air, the shorter telomeres were in children and adolescents. Telomeres get shorter with each year during the aging process but also with increased DNA damage which can be caused by a variety of factors including compounds in air pollution. This was a correlative study so causation can not be established. However, a strength of this study is that the telomere shortening was more aggressive with increasing concentrations of polycyclic aromatic hydrocarbons in the air. Demonstrating that an effect occurs in a dose-dependent manner strengthens any correlation.