Note: This article was first published on 9th September 2015 and it's still relevant today.

It looks like the haze is back and there’s not much we can do except stay indoors, wear masks outdoors, and drink lots of fluids. Another way to cope is to get an air purifier for the home, but knowing which features you need and which you don’t can get confusing. If you haven’t already bought one, here’s what you need to look out for.

1. The most important thing is a HEPA filter

You can skip on everything else, but the one thing you must have in an air purifier is a High Efficiency Particulate Air (HEPA) filter, which can remove more than 99.97% of 0.3-micron particles.

That 0.3-micron figure is important. There are two ways of measuring particle matter (PM): PM10 and PM2.5. PM10 measures particles from 2.5 to 10 microns in diameter, while PM2.5 measures particles 2.5 microns in diameter or less. A human hair has a diameter of 40 microns, so these are extremely small particles that you can’t see with the naked eye. To put it another way; just because you can’t see them doesn’t mean that they’re not there.

PM10 particles can be trapped by your nose or throat, but PM2.5 particles are so small that they can slip through and lodge in your lungs, causing all sorts of health problems.

To trap and filter PM2.5 particles, you’ll need that HEPA filter. Without it, any air purifier is really just decoration. We recognize that air purifiers with HEPA filters can cost more, but if you want to get something to do a job, then you need to know that it actually does that job.

Beware of air purifiers claiming to be “HEPA-type,” “HEPA-like,” or “99% HEPA,” which are not true HEPA filters and not as effective

2. Match the air purifier to the size of your room

The second thing to look for is an air purifier that’s built for the size of your room. If you put an air purifier built for a small room into a large one, it won’t be able to filter air as well.

Some manufacturers list the recommended area in their specifications sheet, which is an easy way to find out if that model fits your room. Some list a number called CADR, which stands for Clean Air Delivery Rate.

CADR is an independent test result from the Association of Home Appliance Manufacturers (AHAM) in the US. The number is expressed in cubic feet or meters per minute, and it measures for tobacco smoke, pollen and dust filtration. The higher the CADR number, the more air it filters per minute for that particle.

Note that CADR doesn’t measure the effectiveness of the size of particles being filtered, that’s where you still need to have that separate HEPA filter specification.

How to get the right CADR rating for your room

AHAM recommends a ‘two-thirds’ rule when it comes to the tobacco smoke rating, that is, you’ll want to find an air purifier with a CADR (tobacco smoke) that is at least two-thirds of your room’s area.

The problem is that the number is mostly listed in square-feet, not square-meters, so you’ll have to do the conversion yourself. For example, if you have a room that is 120 square-feet, you’ll need a CADR (tobacco smoke) rating of at least 80 (two-thirds of the room’s area). Yup, some math is required.

If you truly cannot afford to get a HEPA air purifier sized to your room, then you can consider placing a smaller-rated air purifier closer to the important areas, like near your bed, or near the sitting area of your living room.

3. No ozone

We don’t recommend air purifiers which use ozone to clean the air. Ozone can irritate the lungs and cause chest pains, coughing, and shortness of breath, even in people who don’t suffer from asthma and lung problems. Children and older adults are more sensitive to ozone.

According to the United States Environmental Protection Agency, “At concentrations that do not exceed public health standards, ozone has little effect in removing most indoor air contaminants.” Even low levels can be harmful, and repeated exposure can permanently scar lung tissue.

Many air purifiers sold these days no longer use ozone, but there are air purifiers which use ion generators to clean the air, and they emit low levels of ozone.

Air ionizers (negative ion generators) and ozone

Air ionizers, or negative ion generators, discharge electrons which form negative ions in the air. These negative ions then attach to small particles like smoke, dust and pollen, clumping them into larger particles which can be more easily filtered by the air purifier. However, these larger particles can also simply descend to the ground, or attach themselves to positively charged surfaces in the room.

The problem with negative ion generators is that they typically produce a low level of ozone. Now these may be levels low enough that they won’t affect your health, but we’re not keen to take the risk. If you’d still like to get an air purifier with a negative ion generator, take a look at the air purifier’s ozone output and compare it to the recommended levels.

The United States Environmental Protection Agency (EPA) recommends a maximum eight-hour average outdoor concentration of 0.08ppm of ozone, while the United States Occupational Safety and Health Administration (OSHA) and National Institute of Occupational Safety and Health (NIOSH) both recommend an upper limit of 0.10ppm, not to be exceeded for more than eight hours (OSHA) or at any time (NIOSH) (source: Ozone Generators that are Sold as Air Cleaners: An Assessment of Effectiveness and Health Consequences, PDF link).

4. Listen for the noise of the fan

The gold standard for air purifiers is mechanical filtration, which uses a fan to force air through a HEPA filter to trap particulars, while letting the air flow back out. There are other kinds of air purifiers, but mechanical filtration is proven and cost effective.

However, one by-product of mechanical filtration is noise. If you’re going to be running the air purifier in your bedroom, you won’t want one that sounds like a vacuum cleaner.

So how do you figure out how loud an air purifier is? While you can listen to it in the store, loudness is relative — what sounds quiet in a crowded mall can sound loud in a quiet bedroom. For a more objective test, check the air purifier’s rated noise levels, measured in decibels (dB or dBA). It’s complicated, but to keep it simple for this post, dB and dBA are roughly equal measurements.

For some perspective, Hearing Aid Know has a list of common noises and their decibel levels. Vacuum cleaners are rated at 80dB, normal conversation at 60dB, and a refrigerator humming at 40dB. The Engineering Toolbox recommends that living rooms have a maximum noise level of 50dBA, and bedrooms a maximum noise level of 30dBA.

You should also know that decibels are measured on a logarithmic scale, which means that the difference between values increase as the values get larger. For example, the difference between 20dB and 10dB is not twice, but four times as loud.

5. Buy brands that will stick around

HEPA filters clog up over time with the particles they trap, and you’ll need to replace them every once in a while. It does add up to the cost, but that’s the price of running an air purifier.

You don’t want to buy an air purifier, only to have problems finding replacement filters down the road. That’s the reason we’d recommend you buy an air purifier from a known brand, that will likely be around for the long-term and keep selling its filters down the road.

Wait, what about the other filters?

To keep things simple, we’ve listed only the absolute essentials for an air purifier to deal with the haze. To sum up, an effective air purifier must:

  • Have a true HEPA filter
  • Be the right size for the room
  • Produce zero ozone
  • Use a fan that’s quiet enough for the room
  • Be around for the long-term

Other filters are nice-to-haves in comparison. For example, carbon air filters can absorb smoke, odors, chemicals and gas, while ultraviolet light filters can kill bacteria. If you don’t need these features and just want to deal with the harmful PM2.5 particles in the haze, you can skip them.

3 tips for using an air purifier

Lastly, air purifiers aren’t miracle machines. If you run it in a room with open doors and windows, it won’t be able to keep up with the amount of air that’s constantly flowing in and out.

1. Air purifiers are best used in air-conditioned or closed rooms

Air-conditioners don’t draw air in from the outside, so the air purifier will have a better chance of cleaning the air inside an air-conditioned room.

2. Keep air purifiers well-ventilated

Place the air purifier in a spot where it has good ventilation, which helps make its job easier. Don’t stuff it into a corner where its fans will be blocked and air can’t escape.

3. Run it wisely

You only need to run the air purifier when you’re in the room, or just before you’re going to use the room. It doesn’t need to be switched on 24/7 when nobody is around.

The air purifier needs some time to do its job, so you can run it half an hour to an hour ahead of time before you use the room, like in the bedroom before you go to sleep.

This article was first published on Hardware Zone.