Previous research had unsurprisingly linked high levels of fine particulate matter in breathing air (also known as PM2.5) to lung cancer. A new study links PM2.5 in breathing air to increased risk of several cancers including liver, pancreatic, and breast cancer. For example, women had an 80% increase risk of breast cancer for every 10 µg/m³ increase in breathing air PM2.5 concentration.

The study was conducted by the University of Hong Kong and the University of Birmingham and included 66,820 people aged 65 years old or older, living in Hong Kong and followed over a period of 10 years. During this period, Hong Kong had an estimated annual average of 34 µg/m³ of PM2.5. The World Health Organization (WHO) deems breathing air that averages a concentration of PM2.5 higher than 10 µg/m³ to be unsafe. Several US and European cities exceed this value, including New York city (14 µg/m³), London (16 µg/m³), Paris (17 µg/m³), Amsterdam (18 µg/m³), and Los Angeles (20 µg/m³). Several Asian cities can reach much higher PM2.5 levels including Moscow (22 µg/m³), Beijing (56 µg/m³), Karachi (117 µg/m³), and Delhi (153 µg/m³).

PM2.5, which is mostly a byproduct of combustion by engines such as those of motor vehicles and industrial plants, can cause changes to the cellular levels of a class of epigenetic molecules called microRNAs or miRNAs. Those that are affected by PM2.5 regulate key processes that can be involved in cancer such as regulation of mRNA splicing, miRNA genesis, and the immune system. It remains to be seen if the same miRNAs were affected in the Hong Kong cohort and whether the same miRNAs are involved in all PM2.5-associated cancers (Cancer Epigenetics Society news; May 17, 2016).