[Air pollution (particulate matter and nitrogen dioxide) and skin aging]

It has recently been discovered that air pollution can contribute to skin aging. This conclusion is based on both epidemiological and mechanistic evidence. Specifically, exposure to ambient relevant particulate matter and to nitrogen dioxide (NO₂) is associated with an increased risk to develop facial pigment spots. In addition, genetic studies indicate the involvement of gene-environment interactions because women carrying certain genetic variants of the aryl hydrocarbon receptor (AHR) signaling pathway have a higher risk to develop facial pigment spots in response to exposure to particulate matter (PM2.5). Mechanistic studies prove a cause/effect relationship because topical exposure of human skin ex vivo or in vivo to non-toxic concentrations of a standardized diesel exhaust mixture increased skin pigmentation by inducing melanin de novo synthesis via an oxidative stress response. In line with this, cosmetic anti-pollution products containing anti-oxidants, but also AHR antagonists are effective in reducing or preventing this increase in skin pigmentation. Ultraviolet (UV) radiation is another important environmental factor which can cause skin aging and pigment spot formation. In a real exposure situation, human skin is exposed to both environmental factors simultaneously. Corresponding epidemiological studies show that particulate matter present in the troposphere and solar UV radiation interact with each other. These results emphasize that environmentally induced skin aging results from a highly complex process.