Some states get pollen from trees and grass as early as September, so now is a great time to be looking for a new air purifier or to top up on filters. But what size air purifier do you need?
The problem is many air purifiers have complex spec sheets that can be tricky to navigate. It’s hard to read a CADR number and imagine the size of room that might be suitable for.
Air purifier boxes often say, “suitable for rooms up to X square metres”, but how they arrive at the figure is worth understanding to make sure you don’t buy the wrong size.
What do the numbers mean?
The figures on the box are all useful, but often poorly explained. CADR is the most important one to look at.
What is CADR?
CADR stands for Clean Air Delivery Rate, and it’s a measure of how much filtered air a purifier can deliver per minute, tested separately for smoke, dust, and pollen. The higher the number, the faster the unit clears a particle from the air.
Some newer models also list a PM2.5 CADR number, which is the pollutant most relevant to bushfire smoke and traffic pollution. If you live near bushfire areas or on a busy road, this one is worth looking for.
What is ACH?
ACH means Air Changes per Hour. This is how many times an air purifier filters the entire volume of air in a given size of room within 60 minutes. Most health and safety bodies recommend a number of 5 or more in occupied spaces.
This figure can be calculated by taking the CADR and dividing it by your room volume. If it’s 5 or above, the purifier should work well in your room.
The reason this is valuable is your specific room size is taken into account. A purifier might have an impressive CADR but deliver a mediocre ACH if your room is too large for it. Aiming for an ACH of 5 is a great way to work out what size air purifier you need.
What size air purifier for what size room?
Victoria is one of the worst affected states when it comes to pollen, and the Victorian Government helpfully has a reference table based on a standard 2.4 metre ceiling.
| Room size | Minimum CADR needed |
|---|---|
| 4m² | 48 m³/h |
| 9m² | 108 m³/h |
| 16m² | 192 m³/h |
| 20m² | 243 m³/h |
| 25m² | 300 m³/h |
| 30m² | 363 m³/h |
| 36m² | 432 m³/h |
| 49m² | 588 m³/h |
This table works out to an ACH of about 5. Measure your room’s length and width, find the closest bracket for your area, and round the size up rather than down.
If you’ve got higher ceilings, an open-plan living space, pets, or if you’re near a source of heavy pollution, it’s worth sizing up an extra bracket.
Air purifier features worth buying and avoiding
Marketers love a buzzword to slap on the box, and air purifiers are no exception. Here’s what’s worth looking for and what’s bunk.
What to look for
HEPA grade. Government guidance recommends HEPA filtration graded H13 or higher, meaning it removes at least 99.9 percent of particles larger than 0.3 microns. That includes pollen, dust, mould spores, smoke, and bacteria. In some cases this is labelled as “True HEPA”.
Noise. Air purifiers are designed to run 24/7, so if they’re too noisy, they can become an annoyance that you switch off rather than something that helps you night and day. For a bedroom, look for units that run at 35 to 40 dB. In a living area, 40 to 45 dB is reasonable.
Some brands like Blueair combine HEPA filtration with electrostatic charging that helps them reach a very high CADR without a noisy fan. Quieter units don’t mean lower performance.
Activated carbon. CADR only measures particles, but doesn’t affect gas pollutants or odours. An activated carbon layer helps to reduce smells from cooking, pets, and lingering smoke.
VOCs and formaldehyde from new furniture and plastic products, as well as nitrogen dioxide from gas stoves can be filtered out with an activated carbon layer.
Certifications like the Sensitive Choice blue butterfly show products approved by the National Asthma Council Australia.
What to avoid
Unproven technology. Ionisers, plasma, ozone, and UV technology all sound fancy, but when poorly implemented they can cause irritation that exacerbates asthma and other respiratory conditions. All of these technologies have the potential to generate ozone as a byproduct, which is a known lung irritant.
Ozone can kill pathogens, but only at concentration levels that are unsafe for humans and pets. UV technology has the potential to leak UV-C radiation if not properly shielded, and cold plasma can emit carbon monoxide or nitrogen oxides that are harmful.
Weasel words. While HEPA is worth looking for, some filters are marketed as “HEPA-type”, “HEPA-like”, “HEPA-style”, or “99% HEPA”. All that means is they don’t meet the standard required to display a real HEPA grade.
Practical advice
Your bedroom is a key area. Much of our exposure to allergens happens at night when we’re asleep, so if you can only filter one area you’ll benefit most from a purifier in the bedroom.
Don’t stay on auto during peak months. Auto sensors can lag high pollutant activity by up to 20 minutes, so around October and November, leave the purifier on a higher setting to filter out the most allergens.
Don’t skip replacing the filter. Air purifier filters get filthy, and when they’re full they can recirculate nasty pollutants throughout your home.
Start filtering early. Running the purifier before you start getting symptoms can save you days of sneezing, coughing, and suffering.
Keeping well this pollen season
In most states, peak allergen season lies between September and November, with tree pollen, grass pollen, and even mould in more northern states becoming an issue.
With these tips, you’ll be ready to go shopping for an air purifier if you’re looking to stay prepared rather than playing catch up when you’re already sneezing.
If you’re after a cheaper way to run your appliances like air purifiers, check out our solar guide here.
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