Introduction: The incidence rates of non-melanoma skin cancer (NMSC) and cutaneous malignant melanoma are rapidly increasing. While inhaled particulate matter (PM) has been classified as a lung carcinogen, the associations between ambient PM (either PM2.5 and PM10) and skin cancers have not been studied thoroughly. This study evaluated the associations between PM2.5 and PM10 and incidence of cutaneous malignant melanoma and NMSC of postmenopausal women in the Women’s Health Initiative Observational Study. Methods: Annual particulate matter exposure levels for the subjects were estimated previously from geostatistical spatial correlation models using data from the EPA Air Quality System national monitoring network and the IMPROVE network along with 171 GIS-based geographic covariates. Participants were excluded if they were non-Caucasian, had a previous history of melanoma or NMSC at baseline, had missing air pollution predictions, had missing covariate data, had missing follow-up time, were cases of melanoma or NMSC prior to the first 4 years of follow-up, or were lost to follow-up prior to the first 4 years. Cox proportional hazards models controlling for various sets of covariates, including daily ultraviolet (UV)-B radiation (wavelengths 280 - 315 nm) through watts, were generated to evaluate the relationships between PM and the skin cancers. Sensitivity analyses were conducted to test the robustness of results by using 5-, 7-, and 10 year-PM averages and adjusting for all sun-related covariates. Results: Study participants had a mean follow-up time of 16 years, with a maximum of 23 years. There were 787 adjudicated cases of first occurrence of cutaneous melanoma and 5419 self-reported cases of first occurrence of NMSC. After adjusting for multiple covariates, an increased risk of NMSC by 20% (95% CI: 7 – 36) per 10 μg/m3 increase in PM2.5 was observed. All other effect estimates were consistent with no association of ambient PM air pollution and increased risk of skin cancer. The findings were robust to the multiple sensitivity analyses conducted. Discussion: This epidemiological study is the first study to examine the role of ambient PM air pollution in skin cancer incidence in the United States. With its validated exposure predictions and large sample size, the study findings add to the evidence that PM2.5 exposure may increase the risk for cancers other than lung cancer. However, biases may remain due to potential confounding, exposure measurement error, and self-reporting of NMSC. Further studies are necessary to determine the impacts of co-pollutants, improve estimation of exposure to UV radiation, and examine the effects of PM on NMSC and melanoma incidence in younger women.