When Breathing is Bad for You

Who hasn’t see pictures of air pollution in cities like Beijing and New Delhi where the air is grey and visibility is extremely poor? Air pollution is known to be bad for health, particularly respiratory and cardiovascular health. It is the fifth leading risk cause for world-wide mortality. It has both short-term and long-term effects. It has been linked to millions of emergency room visits worldwide with asthma accounting for anywhere from nine to 23 million globally. Of these five to 10 million asthma visits were linked to fine particulate matter.1

In its “State of the Air 2018" report, the American Lung Association notes that 133.9 million people in the USA are currently exposed to unhealthy levels of air pollution, an increase from the 125 million (or 6.4%) mentioned in the 2017 report.2 It goes on to mention that “more than 4 in 10 people (in the USA) live where the air is unhealthy”. With the current wild fires in California taking air pollution levels to unsafe levels, concerns have escalated about the dangers inherent in increased pollution levels.

A number of factors contribute to outdoor air pollution. Natural causes include smoke from forest fires, wind erosion, and volcanic eruptions. Other causes include vehicle emissions, factory exhausts, refineries, burning fossil fuels, dust, chemicals, mining, agriculture and gases. Indoor pollution is the combination of outdoor air and indoor chemicals released by personal care and cleaning products, heating, painting, renovations, burning of wood, volatile organic compounds, and many others.

The major components of air pollution are ozone, sulfur dioxide, nitrogen oxides (NOx), carbon monoxide, particulate matter (PM) and volatile organic compounds. [For a detailed explanation of the effects of each of these pollutants, see reference 3.]

There are two ways of measuring air quality: the Air Quality Index (AQI) and the Air Quality Healthy Index (AQHI).

The Air Quality Index

The AQI measures the level of pollutants on a scale from 0 to 500. The higher the AQI value, the higher the level of air pollution and the greater the health risk. A level of 100 is the set standard for health, and any level above 100 is considered unhealthy.

The AQI is based on a formula based on the levels of four major air pollutants – ozone, particulate matter, carbon monoxide and sulfur dioxide. The Environmental Protection Agency (EPA) has established standards for each of these pollutants. Currently nitrogen dioxide is not included.4

The computed AQI is colour-coded to represent the levels of air pollution.

AQI range Color Air Quality

0 – 50 Green Good – no health risk

51 – 100 Yellow Moderate but acceptable

101 – 150 Orange Unhealthy for sensitive groups*

151 – 200 Red Unhealthy for everyone

Sensitive groups include children and the elderly, anyone with chronic respiratory disease and people who are active outdoors

EPA recommendations are as follows:

Color Sensitive Groups Everyone

orange reduce outdoor exertion - - -

red avoid prolonged or heavy outdoor exertion limit prolonged outdoor exertion

purple avoid all outdoor exertion limit all outdoor exertion

The Air Quality Healthy Index

The AQHI (used in Canada5), consists of a simple numeric scale from 1 to 10 that represents the amount of air pollution present at any given time and the associated health risk of short-tern exposure to nitrogen dioxide, particulate matter and ground-level ozone. Only these three major pollutants are combined according to a formula to indicate a level of risk. Other pollutants such as carbon monoxide, soot, sulphur dioxide, polycyclic aromatic hydrocarbons, pollen, dust, smoke, volatile organic compounds and odours are not included.

The AQHI is linear, ranges from 1 to 10, and is color-coded from sky blue (safe) to red (dangerous). The colors represent the levels of air pollution. Levels above 10 are shown as black.

According to the AQHI, the health risk for the general population is considered

  • low, when the index is between 1 and 3. This is the ideal air quality for outdoor activities
  • moderate, between 4 and 6. At these levels, activities should be modified if experiencing symptoms
  • high, between 7 and 10. Activities should be reduced or re-scheduled.
  • very high, for levels above 10. All strenuous outdoor activities should be reduced or rescheduled.

While the periods of exposure can vary from a few days to months, the harmful effects of air pollution are most marked for individuals considered “high risk”, such as

  • infants, children, pregnant women and the elderly
  • anyone with chronic respiratory or cardiovascular disease
  • those with co-morbid conditions such as obesity and diabetes
  • people who work or are active outdoors.

The AQHI recommendations for high-risk individuals is different. For levels that are

  • low (between 1 and 3), there are no restrictions
  • moderate (between 4 and 6), activities should be reduced or rescheduled if experiencing symptoms
  • high (between 7 and 10), activities should be reduced or rescheduled
  • very high (above 10), strenuous outdoor activities should be avoided; and elderly and children should avoid physical exertion.

It should be noted that amongst adults, the elderly in particular are more affected by air pollution.

Neither the AQI nor the AQHI measure the health effects of

  • low levels of other pollutants on individuals, or on low levels of pollutants on high-risk individuals who suffer from pulmonary and cardiovascular problems
  • indoor air pollution
  • long-term exposure to outdoor air pollution.

Air pollution and mortality

The World Health Organization (WHO) estimates that, worldwide, 9 out of 10 people breathe polluted air, and 91% of people live in places where air quality exceeds the limits suggested by the WHO guidelines. It estimates6 that 4.2 million people die every year due to outdoor air pollution, which is said to cause 1 in 9 deaths, and a further 3.8 million die from indoor polluted air. Altogether, 29% of deaths will be due to lung cancer, 24% from stroke, 25% from heart disease and 43% from lung disease. Thus air pollution, both outdoor and indoor, will have contributed to more than 8 million deaths.

Calculations of mortality include only five causes of death – ischemic heart disease, stroke, chronic obstructive pulmonary disease, lung cancer and lower respiratory infections. However, a recent publication by Burnett and others7 found that current estimates of the health risks of particulate air pollution are 7-fold lower than their calculations. Their results showed that particulate air pollution may result in additional causes of death that currently are seriously underestimated or ignored. Their calculations suggest a 30% increase in deaths over the current estimate.

Are the current National Ambient Air Quality Standards in the USA too lax and not do enough to protect people’s health? Di and others8 found that there was no safe threshold to either PM2.5 or ground-level ozone. The data on over 60 million Medicare and Medicaid Service beneficiaries 65 years and older were examined with a follow-up of 7 years where death was the outcome. The risk of death, even if these pollutants were below national air quality standards, was high, and even higher for blacks and Medicare beneficiaries, particularly males. Racial minorities and low-income earners were particularly vulnerable. The researchers found that for each increase in 10 µg/m3 of annual exposure to PM2.5, there was a 7.3% increase in mortality. For every 1 ppb increase in ozone, there was a 1.1% increase in the rate of associated mortality.They concluded that a 1 ppb reduction in the ozone level would save 1,900 elderly lives annually.

Air pollution levels below EPA standards have been linked to higher death rates. Researchers found that in areas with high levels of fine PM, the death rates of adults aged over 65 were higher than those in areas of lower PM. The deleterious effects of PM were seen even in areas where the level of PM was less than a third of the current EPA standard9 – the researchers found significant acute and chronic effects of PM exposure at even lower levels.

. . . . to be continued
( Part 2 in this series is titled “The Effects of Air Pollution”)


C carbon, NOx nitric oxides, ppb parts per billion, PM particulate matter <2.5 microns, PAH polycyclic aromatic hydrocarbons


  1. Anenberg SC, Henze DK, et al. Estimates of the global burden of ambient PM2.5, ozone, and NO2 on asthma incidence and emergency room visits. Environmental Health Perspectives, 2018; 126 (10): 107004 DOI: 10.1289/EHP3766

  2. http://www.lung.org/local-content/california/our-initiatives/state-of-the-air/2018/state-of-the-air-2018.html

  3. Govias G. A Breath of Air . . . and what else? The Asthma Education Clinic. 2014. http://www.asthmaed. com/products/a-breath-of-air-and-what-else-air-pollution-and-asthma

  4. https://www3.epa.gov/airnow/aqi_brochure_02_14.pdf.

  5. https://www.canada.ca/en/environment-climate-change/services/air-quality-health-index/about.html#What_is_AQHI

  6. http://www.who.int/airpollution/en/

  7. Burnett R.Chen H et al. Global estimates of mortality associated with long-term exposure to outdoor fine particulate matter. Proc Natl Acad Sci U S A. 2018. pii: 201803222. doi: 10.1073/pnas.1803222115.

  8. Di Q, Dominici F, Schwartz JD. Air Pollution and Mortality in the Medicare Population. N Engl J Med. 2017 Oct 12;377(15):1498-9. doi: 10.1056/NEJMc1709849.

  9. Shi L, Zanobetti A et al. Low-concentration PM2.5 and mortality: estimating acute and chronic effects in a population-based study. Environ Health Perspectives, 2015; DOI: 10.1289/ehp.1409111