WHO annual mean suggests lower safe air pollution threshold

WHO annual means for air polution

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In a previous article, recommended wearing a mask when the US Air Quality Index is at “Moderate” (AQI 51) or higher. But annual air pollution exposure is usually more salient than short-term exposure unless the air quality on a given day is seriously unhealhty. A recent examination of the World Health Organization’s current guidelines for annual air pollution exposure reinforces this. In fact the WHO annual mean limits suggest the need to wear a mask at an even lower threshold than “Moderate.”

The problem with most air quality indexes around the world is that they only consider short-term air pollution exposure. No consideration is given to the effects of long-term exposure. What the WHO has established is that lower amounts of air pollution exposure over a long period of time can also be harmful.

WHO annual mean limit

(except as noted)

US AQI Lowest “moderate air quality” level WHO hourly or daily mean US AQI Lowest “Unhealthy for everyone” level
PM 2.5 (fine particulates) 10 µg/m3 12.1 µg/m3 (24 hour mean) 25 µg/m3 (24 hour mean) 55.5 µg/m3 (24 hour mean)
PM 10 (coarse particulates) 20 µg/m3 55 µg/m3 (24 hour mean) 50 µg/m3 (24 hour mean) 255 µg/m3 (24 hour mean)
Carbon Monoxide N/A 5,152 µg/m3 (8 hr avg) N/A 14,313 µg/m3 (8 hr avg)
Sulfur dioxide 20 µg/m3 (annual mean) 94.3 µg/m3 (1 and 24 hr mean) 500 µg/m3 (10 minute mean) 487.3 µg/m3 (1 and 24 hr mean)
Ozone (O3) N/A 110 µg/m3 (8 hr avg) 100 μg/m3 (8-hour mean) 172 µg/m3 (8 hr avg)

330 µg/m3(1 hr avg)

Nitrogen Dioxide (NO2) 40 μg/m3 (annual mean) 101.5 μg/m3 (1 hr mean) 200 μg/m3 (1 hr mean) 678.6 μg/m3 (1 hr mean) 361 ppb


The US Air Quality Index, put out by the Environmental Protection Agency, defines Moderate air quality (51-100) this way: “Air quality is acceptable; however, for some pollutants there may be a moderate health concern for a very small number of people who are unusually sensitive to air pollution.”

This may be true of immediate reactions people may have, but look how low the WHO annual mean is compared to the EPA “moderate” level.

My personal experience has been that three most common pollutants that push the AQI index the highest are the PM 2.5, PM 10 and ozone levels. If you were to plot the WHO’s annual mean limits for PM 2.5 and PM 10 on the US AQI chart, that annual mean would be exceeded at 42 on the US AQI chart. That is considered in the “Good” range.

But how would you know whether you are exceeding the annual mean of exposure for PM 2.5? You could diligently get air quality readings for every hour you are outside, plot them on a chart and hope they are accurate. But it would be a royal pain.

An easier way to stay under the WHO annual mean for PM 2.5 would be to put your mask on when the US AQI reaches 42. That would allow for times when the readings are inaccurate, or you left your mask at home, or lack a new filter for your mask, or exposure to indoor pollution and the times when outdoor pollution seeps inside. Essentially, it would give the cilia in your lungs some wiggle room.

For you sci-fi nerds, you will note that in The Hitchhiker’s Guide To The Galaxy by Douglas Adams, the number 42 is the Answer to the Ultimate Question of Life, the Universe, and Everything.

As for ozone, the WHO hasn’t established an annual mean. But the WHO limit for the 8-hour mean of exposure to Om is lower than “Moderate” range on the US AQI chart, which is also based on an 8-hour mean. The WHO’s level would read at 45 on the US AQI chart.

Is 42 even enough?

The WHO’s statement regarding the significance of their standards is worth noting, too. The WHO stated, “Small particulate pollution have health impacts even at very low concentrations – indeed no threshold has been identified below which no damage to health is observed. Therefore, the WHO 2005 guideline limits aimed to achieve the lowest concentrations of PM possible.”

In a statement accompanying the WHO guidelines for particulate matter concentrations, they said that their guidelines “estimate that reducing annual average fine particulate matter (PM 2.5) concentrations from levels of 35 micrograms per cubic meter, common in many developing cities, to the WHO guideline level of 10 micrograms, could reduce air pollution-related deaths by around 15%. However, even in the European Union, where PM concentrations in many cities do comply with Guideline levels, it is estimated that average life expectancy is 8.6 months lower than it would otherwise be, due to PM exposures from human sources.”

A fifteen percent reduction in deaths is no small deal when dealing with millions of people worldwide. Note, that death rates are only one measure of the impact of air quality. The number of people whose health declines due to air pollution is even higher. Unhealthy air quality has been linked to heart health, asthma, and even dementia. So you need realize that we aren’t necessarily out of the woods even when the air quality meets WHO guidelines.

WHO set its current standards in 2005 and are in the process of being revised. Revisions will likely be published in 2020. It’s worth noting that WHO has a small budget—just $2 million per year, and 80% of their funding comes from private sources. So while WHO has stricter (albeit non-binding) standards than the US, they are not immune to donor influence. History demonstrates how campaign donations have influenced government. President Obama shelved efforts to tighten EPA air quality standards immediately prior to the 2012 Presidential election, and only tightened them after he won his second term.

So is 42 the answer to life, the universe and everything when it comes to air quality? For now, maybe. But it’s subject to change and could even be lower in the near future.