Thomas, MPhil, PhD, is a reader in epidemiology in the Department of Public Health, Epidemiology and Biostatistics in the Institute of Applied Health of the College of Medical and Dental Sciences at The University of Birmingham.

For this study, Thach, Thomas, and colleagues enrolled 66,280 people who were age 65 or older when initially recruited between 1998 and 2001. The researchers did not have data on whether they had cancer before they were enrolled. Researchers followed the study subjects until 2011, ascertaining causes of death from Hong Kong registrations. Annual concentrations of PM_2.5 at their homes were estimated using data from satellite data and fixed-site monitors.

After adjusting for smoking status and excluding deaths that had occurred within three years of the baseline to control for competing diseases, the study showed that for every 10 µg/m³ of increased exposure to PM_2.5, the risk of dying from any cancer rose by 22%. Increases of 10 µg/m³ of PM_2.5 were associated with a 42% increased risk of mortality from cancer in the upper digestive tract and a 35% increased risk of mortality from accessory digestive organs, which include the liver, bile ducts, gall bladder, and pancreas.

For women, every 10 µg/m³ increase in exposure to PM_2.5 was associated with an 80% increased risk of mortality from breast cancer, and men experienced a 36% increased risk of dying of lung cancer for every 10 µg/m³ increased exposure to PM_2.5.

Thach and Thomas suggested possible explanations for the association between PM_2.5 and cancer could include defects in DNA repair function; alterations in the body’s immune response; or inflammation that triggers angiogenesis—the growth of new blood vessels that allows tumors to spread. In the case of the digestive organs, heavy metal pollution could affect gut microbiota and influence the development of cancer, the authors added.

In 2015, the International Agency for Research on Cancer (IARC) published a series of monographs on the evaluation of various carcinogenic risks. In a monograph on air pollution, the organization pointed out the difficulty of assessing the effects of pollution on multiple types of cancers, given their different etiologies, risk factors, and variability in the composition of air pollutants in space and time.

The IARC also identified certain key components of air pollution, including particulates. The large scale of Thach and Thomas’s study, as well as its documentation of cancer-specific mortality, enables the detailed investigation of the contribution of particulate matter to these cancers, the authors said.

Thomas added that further research would be required to determine whether other countries experience similar associations between PM_2.5 and cancer deaths, but this study combined with existing research suggests that other urban populations may carry the same risks.

The implications for other similar cities around the world are that PM_2.5 must be reduced as much and as fast as possible. Air pollution remains a clear, modifiable public health concern.

—Neil Thomas

Thach said a limitation of the study is that it focused solely on PM_2.5. He said emerging research is beginning to study the effects of exposure to multiple pollutants on human health. He also cautioned that pollution is just one risk factor for cancer, and others, such as diet and exercise, may be more significant and more modifiable risk factors.

This study was funded by the Wellcome Trust. Thach and Thomas declare no conflicts of interest.

Resources

• Chit Ming Wong, Hilda Tsang, Hak Kan Lai, G. Neil Thomas, Kin Bong Lam, King Pan Chan, Qishi Zheng, Jon G. Ayres, Siu Yin Lee, Tai Hing Lam, and Thuan Quoc Thach (2016) “Cancer Mortality Risks from Long-term Exposure to Ambient Fine Particle” Cancer Epidemiol Biomarkers doi: 10.1158/1055-9965.EPI-15-0626