Polycyclic aromatic hydrocarbons (PAHs) are a class of compound containing several known or suspected carcinogenic materials. However, in many large scale and global models of aerosol composition, predicted PAH concentrations are lower than those observed in measurement studies, thought to be because the degradation process of PAHs via reaction with ozone at particle surfaces is not correctly captured. A recent study combining modelling and measurement of benzo(a)pyrene (BaP), often used as an indicator of PAH-related cancer risk and itself a Class 1 carcinogen, shows that increased viscosity of organic aerosol coatings, especially in cool, dry environments, reduces the availability of PAHs for surface reactions which remove them, leading to a longer lifetime for this carcinogenic material within the particles. This ‘PAH shielding’ increases both the distance over which PAHs may travel between source and exposure, and the received dose. This in turn leads to an estimated 4-fold increase in lung cancer risk in both urban and rural locations compared with previous model estimates, with China and India most affected. In the tropics, higher temperature and humidity mean that more oxidised PAH products are found, with as yet not fully understood health implications.

Visit Phys.org for further comment and to view the findings, published this week in the Proceedings of the National Academy of Sciences Early Edition online.