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Asthma,General Health

Air Quality in China (Xi’an)

Xi'an Terra Cotta Warriors

We were honored to be able to present to business leaders and school and hospital administrators in a conference on air quality in Xi’an China recently.  Xi’an, home to the Terra Cotta warriors unfortunately has some of the worst air pollution in China.  As part of an education outreach we presented on the current air situation and provided solutions to help people breathe easier.


There are many factors that contribute to poor air quality and this issue is best summarized by the World Health Organization where they state that air pollution is now the world’s #1 health risk. Many factors contribute to air pollution with the primary drivers being population and economic growth that has led to increased levels of energy consumption. There are also other factors such as the use of formaldehyde in building materials, education on indoor cooking practices and the use and storage of chemicals/cleaning supplies indoors.

First some definitions. Particulate matter are the particles in the air such as dust, pollen, dirt, smoke, etc. PM2.5 is a measure of particulate matter (PM) that is less than 2.5 microns in diameter. These are the smallest particles and they are the most dangerous since they stay suspended in the air the longest and can be breathed deep into your lungs. PM10 are particles that are 10 microns in size and smaller. They are less dangerous as they are more likely to settle to the ground or if inhaled to be filtered out by your nasal passages.

Besides particulates there are also a variety of gases in the air. The most common gases in China are formaldehyde, smog/ozone, sulfur and combustion gases. While these gases don’t get the same press as PM2.5, they can be equally as dangerous and should be removed to provide a safe level of indoor air. The most common being formaldehyde, a colorless gas commonly used in building materials.

So, how do you effectively remove the PM2.5 and spectrum of gases? The short answer is that there is no one technology that is particularly effective for everything. Let’s start first with PM2.5

The most effective technology to remove PM2.5 is a HEPA filter. HEPA filters were first developed in the 1940’s by the US government as a way to create an effective gas mask. Little has changed with HEPA filter media since then and it remains the most effective way to remove particulates from the air. By definition, a HEPA filter removes 99.97% or more of particles that are sized 0.3 microns and larger.

Gas Removal
The removal of a variety of gases indoors is very important in China and this is an area where nearly all air purifiers fall short. The challenge is that gases are too small to be captured by HEPA filters and the most effective technology is a high quality granular activated carbon (GAC). Most air purifiers have little to no carbon because the products are too small to hold a sufficient amount of carbon or the motor is too weak to move the air through a dense carbon filter. Those that have more than a couple kg of carbon often use low grade carbon or carbon that is not treated to remove formaldehyde and other difficult to remove gases.

Granular activated carbon (GAC) are shaped like small pellets and to be effective the filter depth of GAC should be 0.8” or more. If you are looking at the filters in a store we suggest holding the filter up to the light and if you can see through the carbon filter it’s performance will be limited.

In addition there are various grades and quality of carbon and it is almost impossible for a shopper to know good from bad. Manufacturers often promote very high performance rates like 99% removal however currently the performance standards do not exist to make a fair assessment across the products. Our suggestion is to ask where the carbon is made, how much carbon is in the filter and the depth of the carbon.

This is an area where we are pushing for standards with the goal of ensuring you know what they are buying.

Putting It All Together
So, to effectively clean the indoor air in China it is best to have a large HEPA filter with a high rating (ie H13) and an ample amount of high quality activated carbon (GAC). Nearly all of the air purifiers that are sold in China were developed to address air quality issues in America and Europe and the problem is their environments are very different.

In American and Europe the primary issue is allergies related to dust, pollen and mold so they do not require the same high quality HEPA filter and do not have the same need for carbon for the gas removal (formaldehyde, ozone, smog, combustion gases and sulfur).