The World Health Organization (WHO) released revised air quality guidelines on September 22, 2021, the first major update to the standards in 15 years.¹

The revised recommendations, produced from evidence-based data and information and a growing awareness that no amount of air pollution is safe for human beings, resulted in nearly all pollutant standards being strengthened in comparison to the air quality guidelines established in 2005 (published in 2006).² If target levels of pollution are implemented by governments, these new guidelines will save lives by reducing preventable airborne pollutant deaths.

The WHO noted that updating the newly updated air quality guideline (AQG) levels was motivated by a desire to save lives. The guidelines are intended to spur greater global reductions in pollution emissions.

Some of the updated key air pollutants also contribute to human-induced climate change, which can create conditions for deadlier wildfires. Wildfire can, in turn, result in greater exposure to airborne pollutants.

Revisions improve key pollutant air quality standards

All key pollutants listed in the 2005 guidelines were reviewed and revised.

Key pollutant guidelines that were revised included:

• PM2.5
• PM10
• ozone
• nitrogen dioxide
• sulfur dioxide
• carbon monoxide

Each pollutant received a recommended AQG level based on an average time, typically annually or over a 24-hour period. While nearly all of the recommended levels were strengthened, a few air quality guidelines from the 2005 update and later 2010 indoor air quality guidelines weren’t re-evaluated or revised.³

Here’s a pollutant-by-pollutant look at the guideline changes.

Particulate matter

Airborne particulate matter (PM) is found in dust, soot, dirt, and smoke. Power plant combustion, vehicle combustion, and wildfires are major sources of PM.

Pictured: Air quality index measurements displayed across the globe provide a visual representation of toxic airborne pollutants. Source: IQAir

Guidelines for coarse and fine particulate matter, PM10 and PM2.5 respectively, were updated for annual and 24-hour averaging times. PM2.5 are particulates measuring 2.5 micrometers (µm) in diameter or less. PM10 are larger, measuring 10 micrometers in diameter or less.

In 2005, the highest recommended average annual emission level for PM2.5 was 10 μg/m³. The 2021 revision halves that number, to just 5 μg/m³. The 24-hour level changed from 25 μg/m³ in 2005 to 15 μg/m³.

The recommended average annual emission level for PM10 was 20 μg/m³ in 2005. The level for 2021 shifted to 15 μg/m³. The 24-hour level was updated from 50 μg/m³ in 2005 to 45 μg/m³.

The WHO determined that there was not enough data to provide guidelines for other types of PM, including black carbon and elemental carbon, sand and dust storm particles, and ultrafine particles (UFPs) – aerosolized particles measuring as little as little as 0.1 micrometers in diameter or less. But the organization did create a set of best practices for managing those pollutants while recommending further study into their individual risks and mitigation methods.

Ozone

Tropospheric ozone, or ground-level ozone, is both a pollutant and a greenhouse gas. It’s created by the interaction of nitrogen oxides and volatile organic compounds (VOCs). Tropospheric ozone is the primary ingredient in smog.

The WHO established a recommended AQG level of 60 μg/m³ during “peak season.” The standard is new for 2021. Peak season refers to the six consecutive months with the highest running-average of ozone concentration. Within that timeframe, mean ozone concentrations are averaged over a daily maximum of 8-hours.

Nitrogen dioxide

Most nitrogen dioxide in the environment comes from burning fuel in vehicles and power plants. Nitrogen dioxide is a component in forming tropospheric ozone.

WHO guidelines were sharply revised from 40 to just 10 μg/m³ on average annually. A new standard level of 25 μg/m³ on average per 24-hour period was also recommended.

Nitrogen dioxide guidelines of 200 μg/m³ on average per hour are still valid.

Sulfur dioxide

Nearly all sulfur dioxide emissions come from human activity and industrial activity.

The WHO’s sulfur dioxide recommendations were to raise the level from 20 μg/m³ to 40 μg/m³ in a 24-hour period – the only pollutant level to ease in 2021.

Sulfur dioxide guidelines of 500 μg/m³ on average for a 10-minute interval weren’t re-evaluated and remain the recommendation.

Carbon monoxide

Carbon monoxide is an odorless, colorless, and tasteless gas caused by incomplete combustion. The gas is poisonous; carbon monoxide poisoning can kill people.

The recommended level of 4 mg/m³ averaged over a 24-hour period is new to the 2021 guidelines.

Several previous carbon monoxide standards weren’t changed, including those for an 8-hour, 1-hour, and 15-minute interval (10, 35, and 100 mg/m³ respectively).

Policy implications

The WHO air quality guidelines aren’t a legally binding standard or document. However, countries and legislative bodies regularly refer to the WHO guidelines when setting airborne pollutant control legal policy.

The new air quality guidelines were determined by six systematic reviews that took over 500 papers into account.

The new air quality standards will challenge governments to do much more to protect the health of their citizens. According to the 2020 World Air Quality Report, half of European cities exceeded the WHO’s target – the old, 2005 target of 10 μg/m³ – for annual PM2.5 pollution. 38 percent of U.S. cities failed to meet the air quality guidelines, and Chinese citizens faced PM2.5 exposure at three times the target recommendation.

Health implications

Air pollution, even at low concentrations, is one of the leading environmental threats to human health.

PM2.5 causes as many as 7 million preventable deaths every year. The WHO projects that if the world were able to meet these new guidelines, approximately 80 percent of PM2.5-related preventable deaths could be avoided.⁴

While all pollutants pose a health threat to human beings, PM2.5 and ultrafine particles are the most dangerous. Due to their tiny size, once inhaled they can embed in the lungs, enter the bloodstream, and harm every organ.

Common health problems and diseases caused by PM2.5 and UFPs include:^5,6

• heart and lung disease
• bronchitis
• emphysema
• asthma and more intense flareups
• early death

The takeaway

The strengthened air quality guidelines are intended to curb preventable deaths. Acting on the recommendations is an urgent next step in reducing disease associated with airborne pollutants.

While it’s important that policymakers quickly step up to meet or improve on the revised target levels, individuals and organizations can also act to help clean the air.

To help reduce air pollutant dangers, individuals can take several steps, including:

• look for ways to reduce pollutant emissions, including burning less fuel
• telecommute, carpool, vanpool, bicycle, or walk when possible
• eat lower on the food chain; choose vegetables and plants over meat a day or more during the week
• monitor your air quality and share air quality data as a public station
• run a high-efficiency home air purifier or clean the air in your personal space

Organizations can help too. Learn how to become a clean air facility or clean air school.