How we estimate burden of disease

The State of Global Air project reports on the burden of disease associated with exposure to PM2.5, ozone, and household air pollution, and the combined impact from all three pollutants (referred to as “total air pollution”). The estimates on this website draw from the most recent evidence from the Global Burden of Disease (GBD) 2017 project of the Institute for Health Metrics and Evaluation at the University of Washington in Seattle.

For the first time, the State of Global Air also reports on the decrease in life expectancy attributable to air pollution. These findings are based on the published work of Assistant Professor Joshua Apte at the University of Texas, Austin, using GBD data and methods from 2016.

The burden of disease and life expectancy methodologies used for the State of Global Air are summarized below. More details can be found in the Report.

 

What is “burden of disease”?

Burden of disease is a term that reflects the health toll imposed on individuals who are sick or disabled from disease, but that scientists use to characterize the impact that illness and death impose on society as a whole. 

It is measured in terms of the numbers of deaths or years of healthy life lost from disease in a population.

 

What health effects are included?

The first step in estimating the burden of disease associated with air pollution is identifying which health problems are likely to be caused by exposure to each of the individual air pollutants.

Many studies conducted over several decades have documented a wide range of ways that PM2.5, ozone, and household air pollution affect our health. Most of these effects result from exposure to air pollution over a long period of time and involve chronic, noncommunicable diseases like cardiovascular (heart) disease, stroke, respiratory (lung and airway) disease, type 2 diabetes, and lung cancer. Air pollution also causes increases in respiratory infections, like pneumonia. Some of these effects are short term, for example, when high-pollution days trigger asthma symptoms or cause a spike in hospitalizations related to respiratory or cardiovascular diseases. Learn more about both the short- and long-term health effects of air pollution here


The World Health Organization, U.S. Environmental Protection Agency, International Agency for Research on Cancer, and other organizations have systematically reviewed the scientific evidence to understand which health problems can be confidently attributed to each pollutant.

The GBD analysis currently estimates the burden of disease from long-term exposures to outdoor PM2.5, household air pollution, and ozone for the health effects:

 

      Outdoor PM2.5

 Household Air Pollution

      Ozone

  • ischemic heart disease
  • stroke
  • lower-respiratory infections (e.g., pneumonia)
  • chronic obstructive pulmonary disease
  • lung cancer
  • diabetes* 
  • ischemic heart disease
  • stroke
  • lower-respiratory infections (e.g., pneumonia)
  • chronic obstructive pulmonary disease
  • lung cancer
  • diabetes*
  • chronic obstructive pulmonary disease

 

 


 

 

* new in GBD 2017

 

What other information is included?

Building on a fundamental understanding of how each of the pollutants affects health, researchers quantify the burden of disease from each air pollutant based on:

  1. estimates of exposures to PM2.5, ozone, and household air pollution, relative to theoretical minimum risk levels,
  2. mathematical functions, derived from epidemiological studies, that relate different levels of exposure to individual pollutants to each cause of death or disability, and
  3. estimates of the underlying rates of death for each of the diseases that have been linked to air pollution.

 

How is burden of disease described?

The GBD project describes the burden of disease attributed to air pollution in terms of two major factors:

  1. Deaths: Deaths in a given year that likely occurred earlier than would be expected in the absence of air pollution.
  2. Disability-adjusted life-years (DALYs): Years of life lost from early death plus years lived with a disability (such as paralysis from a stroke) related to air pollution exposure.

The GBD project also provides data on rates of death and DALYs for a standard population (e.g., deaths per 100,000 people) and age structure that make it easier to compare burden estimates among countries with different population sizes and ages.

 

How are impacts on life expectancy estimated?

Life expectancy is a statistical estimate of the average lifespan (that is, years from birth to death) that an individual in a population is expected to live. It takes into account all of the factors that affect life expectancy in that population, including age and sex, underlying health status, smoking, diet, and other risk factors like air pollution. 

To assess the impact of a particular factor on life expectancy, researchers first quantify the likelihood of dying from diseases related to that factor at different ages in a population. They then calculate a “risk-deleted” life expectancy — that is, the length of life that would be expected if the factor were absent. The average difference between life expectancy with and without the risk factor quantifies the average change in life expectancy that can be attributed to that factor.

More details on the methods used to estimate the loss of life expectancy attributed to air pollution reported in the State of Global Air can be found in Dr. Joshua Apte’s paper.