Pollution and life expectancy

Thursday January 22 2009

“Cuts in air pollution in US cities over recent decades have added an average of five months of life to their inhabitants”, BBC News reported today. It said that a study had matched air pollution in 51 cities with the life expectancy of their inhabitants between 1980 and 2000. It found that people in cities with the biggest shift from polluted to clean air had an average 10 months longer lifespan.

This study has some limitations, including the fact that it looked at populations rather than individuals. As such, it cannot conclude that an individual living in an area with high pollution will have a shorter life than someone living in a cleaner area. However, it does provide evidence of a general link between particulate pollution and life expectancy that needs further study. The study found that for every decrease of 10 micrograms per cubic metre of particulate pollution, life expectancy increased by more than seven months.

Where did the story come from?

Dr C Arden Pope III from the Department of Economics, Brigham Young University in Provo, Utah, and colleagues from Harvard carried out this research. The work was funded by a number of grants from research agencies including the Association of Schools of Public Health, the Harvard Environmental Protection Agency, Particulate Matter Center, the National Institute of Environmental Health Sciences and funds from the Mary Lou Fulton Professorship, Brigham Young University.

The study was published in the (peer-reviewed) medical journal, New England Journal of Medicine.

What kind of scientific study was this?

The researchers say that fine-particle air pollution has been associated with increased disease. In this ecological study, they wanted to see if improvements in air quality resulted in measurable improvements in human health and life expectancy.

To investigate this, the researchers evaluated the changes in life expectancy associated with changes in fine-particle air pollution that occurred in selected counties of the US during the 1980s and 1990s.

This was an ecological study, and so the researchers did not collect data on the exposures and health of individuals. Instead, they focused on life expectancy and particle pollution in metropolitan areas of the US. Data for 61 areas were obtained from the US Environmental Protection Agency, which had sampled particulate matter in the air from 1979 to 1983. These samples were used to calculate the mean (average) concentrations of particulate matter with a diameter less than or equal to 2.5 μm (PM2.5), equivalent to one-twentieth of the width of a human hair. These fine particles can travel deep into the lungs, and have been linked with the worsening of asthma and heart disease.

The researchers also had access to census data on mortality in each of the counties. This was used to estimate annual life expectancies for those areas. Life expectancy was then correlated to air quality data in the 51 metropolitan areas that had data on both variables.

The researchers also collected socioeconomic and demographic data for each county and adjusted their analysis to take into account any factors that can affect pollution or life expectancy. This adjustment took into account other factors recorded in the census that may have changed over time, including income, ethnicity, age, and education. The researchers did not have data on smoking for these years so instead used death rates for lung cancer and bronchitis (COPD) as indirect measures of smoking.

What were the results of the study?

The researchers report that a decrease of 10 micrograms per cubic metre in the concentration of fine particulate matter was associated with a significant estimated increase in average life expectancy of 0.61 years (P = 0.004) (just over seven months). Taking into account socioeconomic or demographic factors, or measures of indirect smoking did not significantly alter this result. Restricting the analysis to larger counties also did not have an impact on the overall result.

In some of the previously heaviest-polluted cities, such as Pittsburgh and Buffalo, the fall was close to 14 micrograms per cubic metre, accounting for the 10 months of increased life expectancy quoted in the newspapers.

Overall, 15% of the increase in life expectancy in the study areas could be attributed to reductions in air pollution.

What interpretations did the researchers draw from these results?

The researchers conclude that a reduction in exposure to fine-particulate air pollution between 1980 and 2000 has contributed to a “significant and measurable improvement in life expectancy in the United States.” They say that this is good news, but acknowledge that there are many other factors that also affect life expectancy.

What does the NHS Knowledge Service make of this study?

This study provides evidence of the link between particulate pollution and life expectancy through looking at large population data. The study has some limitations that have been noted by the researchers, many of which can be attributed to its design.

  • The researchers say they were unable to evaluate the associations to a greater depth (for example by including all 200 counties for which they had mortality data) as there was insufficient data on air pollution for all these counties. They were also restricted to the years that air pollution data were collected and because of incomplete data they were unable to make the association with the high pollution years more specific.
  • The researchers were unable to fully adjust for potential confounders (other factors that could contribute to differences in mortality). These factors, such as smoking, access to health care, healthier diets or lifestyles, were also not collected for whole counties at that time. However, they say that other studies that have managed to adjust for these have found similar results.
  • The study was not conducted using measurements on individuals. This means that it is prone to the ‘ecological fallacy’, a potential error in the interpretation of statistical data where it is assumed that individual members of a group have the average characteristics of the group at large. As such, it is not possible to say that all individuals living in the areas with higher particulate air pollution would have shorter lives than people living in less polluted areas.

Despite the limitations of an ecological study, the evidence here does support efforts that are being made to reduce particulate air pollution in the US. There is no reason to doubt that similar effects would be seen in the UK.

Analysis by Bazian
Edited by NHS Choices