BBC Office Purifier Setup

HEPA Filters Improve Productivity in the Office

We know that air pollution is bad for our health, causing heart attacks, strokes, and lung cancer. But how about the more day-to-day impacts of air pollution including productivity, tiredness, and alertness? A recent Harvard study shows that poor indoor air quality is affecting employees more than once thought, and … Read more

WHO Updates Air Quality Guidelines, But Are They Safe?

After over 16 years, the World Health Organization (WHO) updated its air quality guidelines, including its recommended limits for one of the world’s biggest killers, PM2.5 air pollution. The WHO lowered their annual PM2.5 limit by half, from 10 micrograms to 5 micrograms per meter cubed.  This comes to welcome … Read more

air conditioner at home

Does the Air Conditioner Bring in Dirty Air From Outside?

A question that I get asked often (and that I have always wondered about) is whether my wall-mounted air conditioner is bringing in dirty air from outside. If so, it’d be safer not to use it, especially on days with bad air.

air conditioner

My short answer is no. To explain why, I’ve got three points of evidence:

1. How air conditioners work

Regular wall-mounted air conditioners in China do have a unit outside connected with tubes to the inside, but that tube is not bringing in outside air. It’s passing coolant, and letting heat escape outside.

So where does the air it’s blowing come from? If you look around your air conditioner, you’ll probably discover that it works like mine: it brings air from the top, runs it over the cooling coils, and blows it out the front. It’s recycling indoor air, not bringing in outdoor air.

2. Tests of the air coming out of the air conditioner

I’ve held my particle counter up into the air coming out of my AC unit, and it’s no different from the ambient room air (See a live test here). I’ve also compared that air to outside air on very dirty days, and the air coming out of the AC is nowhere near as dirty as outside air.

AC air quality test
AC air quality test

(I did this test when I had just turned on my AC. If the AC were bringing in dirty air and I were to test the exhaust after I had been running the AC for a long time, then my whole room would be dirty, not just the exhaust.)

3. Tests of the ambient room air before and after turning the AC on

Results? AC makes basically no difference.

Here’s what happened in one test after turning the AC on:

PM2.5 levels after turning on AC

And here is the average effect over 7 different tests in my bedroom.

% change in air quality after turning on AC

In each test, I ran my particle counter for 30 minutes to get a baseline. Then I turned on the AC for 30 minutes. Here I’m comparing the numbers just before I turned the AC on and 30 minutes later. As you can see, there’s basically no effect. If anything, the larger 2.5 micron particles go down slightly. My guess is this is because the coarse plastic filter in the AC unit captures up some large particles.

Conclusion: If it’s hot outside, don’t sweat it. Use your AC.

Central AC: I should note that these tests are of wall-mounted AC units in China. Central air conditioning may work differently.

For those interested, I’m pasting the detailed data below.

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Has Pollution in China’s Major Cities Increased in 2021?

China’s economy has rebounded in 2021 from major slowdowns due to COVID-19 the previous year. But has the economic rebound come at the cost of more air pollution and worse air quality in China’s major cities? Smart Air analyzed PM2.5, PM10, and ozone pollution data for 10 major Chinese cities … Read more

Delhi smog tower

Delhi Unveils Smog-Fighting Purifier Tower, But Does It Work?

Delhi is one of the most polluted cities in the world. PM2.5 pollution levels reach more than 10 times the World Health Organization’s annual limit. To combat air pollution, the Delhi government recently installed a new smog tower in the center of the city. But will this Delhi smog tower have an effect … Read more

Bangalore Gets Its First Smog Tower, But Does It Work?

Although Bangalore is one of India’s fastest-growing cities, this has come at the cost of more air pollution. According to Greenpeace South Asia in 2020, an estimated 12,000 people in Bangalore died due to air pollution. To help improve Bangalore’s air pollution, the city has installed its first smog tower. … Read more

The Dangers of High Carbon Dioxide (CO2) Levels

Although carbon dioxide (CO2) is a byproduct of many natural processes, high levels of CO2 can cause a variety of health effects on humans. In this article, we explain at what levels CO2 becomes dangerous and the negative health effects of these high CO2 levels. Also, how do CO2 levels … Read more

ASHRAE Recommended Air Changes Per Hour

Data shows that improving indoor ventilation can reduce the risk of virus transmission, and is recommended by the CDC. In this article, we’ll cover the ASHRAE recommended air changes per hour for offices, homes, schools, residential and hospitals, using the ASHRAE 62.1 and 62.2 guidelines. ASHRAE Recommended Air Changes Per … Read more

Smart Air is a social enterprise that creates simple, no-nonsense air purifiers and provides free education to protect people’s health from the effects of air pollution. We are proud to be the only certified B-Corp dedicated to fighting air pollution.