The importance of susceptibility
To understand and interpret the observed respiratory health effects, it is crucial to acknowledge the relevance of susceptibility (or protective) factors that modulate individual reactions to exposure to ambient pollutants. The identification of susceptibility factors is subject to intense research. Given the relevance of the pathophysiological mechanisms mentioned previously, it is not surprising that an increasing number of studies report stronger effects of air pollutants in subjects with limited capacity to defend against oxidative stress and to balance inflammatory responses. Such modulating factors may relate to sex, age, underlying diseases and pro- and anti-oxidant intake, as well as a range of genetic characteristics. For example, a controlled trial in Mexican children not only confirmed the association between O3 and respiratory health, but also revealed interactions related to oxidative stress pathways: children with anti-oxidant treatment were far less affected by O3 than the placebo groups; and children with functional variants of the GSTM gene were protected against the adverse respiratory effects of ozone.