Emissions

Air Quality Index – What in the world is that?

Issue 4 and Volume 121.

By Emily Robbins, PE, Burns & McDonnell

The photos are startling: large cities like Beijing and New Delhi enveloped in a gray haze; people wearing face masks in an attempt to protect their lungs; empty playgrounds on days when children must be kept inside because pollution levels are so high. The smog red alerts in mainland China have forced factories, schools, and roadways to shut down. These red alerts indicate that the air is “hazardous,” but what exactly does that mean? How are these levels calculated and how do pollution levels around the world compare with levels in the United States? The answer is a bit more complicated than comparing the red alert level of one country to another.

Air quality indices (AQI) are used around the world to communicate current or forecasted pollution levels to the public. Often described as colors, such as green, yellow, orange, red, etc., the colors correspond to a numerical index value. However, these index values are not directly comparable from one country to the next because government agencies use different calculations. It’s not uncommon to see AQI values in China as high as 600-900, but in the United States the AQI tops out at 500. In China, an individual score is awarded to each of the six pollutants tracked as part of their index (ozone, CO, SO2, NO2, and particulates such as PM10 and Pm2.5) and the final AQI is the highest of these scores. The individual pollutant scores are calculated by a formula published by China’s Ministry of Environmental Protection. In contrast, the US EPA calculates an AQI for each of the same six pollutants, but does not use an overall AQI score and uses different formulas. The governments of Canada, India, Britain, Mexico and South Korea also calculate AQI values, but each uses their own scale and methodology to rate air quality.

Global Map of Modeled Annual Mean Pm2.5 Concentration (μg/m3)

Many people around the world face serious air pollution issues. In fact, the World Health Organization (WHO) recently published a study that estimates 92% of the world’s population lives in locations where outdoor air quality fails to meet the WHO guidelines. Air pollution levels in India recently surpassed those in China, and the two countries are roughly tied for most premature deaths due to air pollution. Because AQI values are not directly comparable, a more accurate way to compare pollution levels around the word is to view them on a concentration basis. The map on this page shows average predicted Pm2.5 levels in micrograms per cubic meter (μg/m3 – mass of pollutant per volume).

The WHO standard for Pm2.5 is 10 μg/m3. High levels of Pm2.5 are particularly dangerous to human health because the small particles can travel deep into the lungs and even enter the bloodstream. Fine particles and ozone are the main contributors to smog and reduced visibility. Many countries with high levels of PM and other air pollutants lack rigorous environmental regulation or rely on sources of energy with higher emissions (and few emissions controls) due to economic realities.

Since the Clean Air Act was passed in 1963, the US has made large strides in improving air quality. Many areas once classified as “nonattainment” (exceeding ambient air quality standards) have worked to improve air quality and are now classified as “in attainment.” Before the Clean Air Act was passed, levels of pollution in some areas of the US rivaled those in heavily polluted areas of the world today.

Industry and regulatory agencies have found ways to work together to improve air quality in the US. These two groups don’t always agree on the appropriate level of regulation, but constructive dialogue between stakeholders helps agencies craft regulations that improve air quality and allow for economical operation. The success story of improved air quality in the US can serve as a model for other countries looking to improve the health and well-being of their own populations.