Common misconceptions about air purifiers

Our "old friend" is back in town: Beijing on Wednesday issued a yellow alert for air pollution, with smog forecast to last for the next three days. It is the first smog alert in the city since July. Tianjin and Hebei followed suit.

In an attempt to cope with the latest emissions, many people are thinking of buying a purifier to help clean the air in their homes.

A well-known magazine in China has listed some of the most common misconceptions about air purifiers.

1 Filter screens of air purifiers are different

Not necessarily.

Most purifiers on the market have the same kind of air filters known as HEPA (High Efficiency Particulate Air), first designed by the United States Department of Energy in the 1940s. The filter has remained largely the same for more than half a century in basic design and function.

All HEPA filters have to be authenticated by the US Department of Energy. Companies claiming to use "HEPA-kind" or"HEPA-like" filters are playing a word game to cover their illegitimate products.

2 Air filters are washable

HEPA filters are designed to trap small particulates not normally captured by others. They have a special physical structure and chemical properties which are not restored by washing.

3 Electronic air purifiers are better

Electrostatic attraction is used to trap polluting particles and remove them from the airflow. They show high initial efficiency, largely because of their ability to remove fine particles. However, electronic purifiers have been known to release ozone, a pollutant that can cause serious health problems to those exposed even to fairly low levels.

4 Foreign-branded purifiers can deal with air pollution in China

Maybe not.

Many foreign products are designed for countries with strict laws governing air pollution and cleaner outdoor air. In some countries, doors and windows have become natural air filters. But in China, the opposite often occurs, making the needs of Chinese customers different from their foreign counterparts.

5 Air purifiers work better in a well-sealed room

Wrong.

Although sealed rooms may avoid exterior pollutants, carbon dioxide exhaled by humans will accumulate in the enclosed space and pose threat to health. Air purifiers can't reduce the concentration of carbon dioxide.

6 Living on a higher floor will help avoid pollutants on the ground

Not the case.

Research shows that the density of PM 2.5 varies little within kilometers above the ground. On the other hand, rooms on higher floors get more air through doors than windows, which means those living on higher floor will take in more dirty air expelled from lower floors.

7 Can air purifiers serve their purposes at all in China?

Not sure.

No matter how clean the air is at home, one can't avoid going outside and being exposed to polluted air. No study has yet proved what damage this drastic difference in air quality can cause to a human body.

Purifiers are generally worsening the country's air. Their production and use consumes electricity. A big part of China's energy supply comes from thermal power, a major cause of air pollution and climate change.

So, what is the most important standard for an air purifier? The CADR (Clean Air Delivery Rate) may be the most recognized. It is a figure representing the volume, in cubic feet per minute, of air that has had all particles of a given size removed. But a high CADR often brings more noise and higher prices.