Background/aim Development of facial pigment spots is generally believed to result from chronic exposure to solar UV radiation. More recently, it was discovered that exposure to traffic related air pollutants including PM is also associated with more pigment spots. Since humans are exposed to a combination of UV and PM, we here investigated the combined effect of PM and UV exposure on pigment spot formation.
Methods We studied 799 women (mean age 73.5 years) of the SALIA cohort study using the SCINEXATM skin ageing score with pigment spots on cheeks and forehead. Long-term exposure to PM2.5, PM10, PMcoarse and PM2.5absorbance at residence was estimated with ESCAPE land use regression models. 5 year mean exposure to UV was obtained using the satellite data (AURA/NASA) and global radiation data from the German climate centre to yield higher resolution:
daily UVB exposure based on the whole daylight period and
UV index based on the hour per day with maximal UV (proportional to the intensity of UV–radiation causing sunburn).
Associations between PM or UV with facial pigment spot formation and the interactions PM*UV were estimated in single and multi-pollutant multiple linear regression models.
Results UV exposure was associated with more pigment spots on the cheeks (7.6% higher score on average; 95% CI: 2.0% to 13.3%) and this effect was only slightly attenuated when adjusting for air pollution (PM2.5: 5.0%; 95% CI: −1.2% to 11.3%). Similarly, the association between PM and pigment spot formation was still visible after adjusting for UV exposure (e.g. adjusting for UV index: PM2.5 attenuation from 7.9%; 95% CI: 2.5% to 13.4%, to 5.9%; 95% CI: −0.2% to 11.9%). The interaction analysis showed that the association between UV and pigment spots depends on the level of PM. At lower PM levels, the UV effect was increasing (e.g. UVBxPM10 p(interaction)=0.0069). Vice versa, at lower UVB exposure levels, the PM effect was increasing. All of these interactions indicated linear dose-responses.
Conclusion To the best of our knowledge, this is the first study to suggest that a hallmark of extrinsic skin ageing, i.e. facial spot formation, is the consequence of a complex interplay of at least two ubiquitous, ambient relevant factors. The adverse effects of UV on human skin might be shielded by high levels of PM and vice versa.
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