INTRODUCTION
Increased awareness of the hazardous consequences of air pollution has led to global efforts to reduce air pollution and improve air quality. Air pollution is a serious issue for overall public health as it has been linked to ~7 million deaths annually, primarily through cardiovascular and pulmonary diseases. As such, it is the world’s largest environmental health risk putting it in the same league with other health risks including smoking, high cholesterol and obesity.1 Air pollution also has aggravating effects on skin conditions and diseases such as atopic dermati- tis.2-4 Various organic and inorganic matter make up air pollution including fine particulate matter (defined by a diameter ≤2.5 μm or ≤10μm as PM2.5 or PM10, respectively) that can penetrate the skin, gases (such as nitrogen dioxide, carbon monoxide, and ground-level ozone), and toxic chemicals (for example, polycy- clic aromatic hydrocarbons (PAH) and dioxins). While the current understanding is that various environmental factors contribute to extrinsic skin damage and aging, the primary research focus has historically been on solar radiation, especially UV exposure.5 However, the past decade has seen increased research efforts to determine the mechanisms through which pollution could cause skin damage as well as its effects on skin aging. In 2010, an epidemiological cohort study following 400 women revealed a significant correlation between air pollution exposure and signs of skin aging, specifically pigment spots and wrinkles.6 A multicenter clinical study in Mexico, including subjects from a high pollution area and from a low pollution area, described a higher frequency of certain skin conditions in the population exposed to more pollution. Interestingly, skin swab and corneo- adhesive disc samples taken in this study also revealed several changes in biochemical parameters including decreased squalene and ATP levels as well as an increase in carbonylated proteins suggesting increased oxidative stress due to pollution.7 Several mechanisms through which air pollutants cause skin damage and aging have been proposed including generation of free radicals and reactive oxygen species (ROS) resulting in oxidative damage to lipids, proteins and DNA. For exam- ple, ozone is highly reactive with skin surface lipids causing lipid peroxidation, which results in cascading inflammatory and oxidative effects in the deeper skin layers, and depletion of surface antioxidants such as squalene.8,9 Squalene is the major scavenger of ozone in the skin and oxidized squalene (squalene monohydroperoxide; SQOOH) is a biomarker for environmental pollution.10,11 Particulate matter (PM) was found
