How effective are indoor air purifiers against wood heater and bushfire smoke?
In some parts of Australia, smoke from domestic wood heaters in winter can also harm our health and has been linked to premature deaths.While relatively few households burn wood for heat in Australia, the smoke given off can affect many more.
Like smoke from bushfires, or controlled burns at other times of the year, it can seep through cracks in your home and build up inside.
From making the most of your indoor air purifier to other simple steps you can take, we spoke with top air-quality experts about how to reduce exposure to smoke pollution from external sources in your home.
While burning wood might smell relatively "natural", that doesn't mean inhaling it is good for you.
Beyond its immediate effects on our lungs and heart, smoke exposure is linked to a range of other problems from gestational diabetes to eye conditions.
A major focus of concern are tiny particles, generated by combustion, called PM2.5. These are just 2.5 microns (or 2.5 millionths of a metre) wide or less, and include the smaller "ultrafine particles", which are less well understood but of growing concern.
Smoke particles can be absorbed by the blood stream, activating the immune system and causing inflammation. They can even enter the brain and may affect cognitive abilities.
People with heart or lung conditions such as asthma, pregnant women, older people and young children are at particular risk from the health effects of smoke inhalation.
The latest research shows just how variable the effectiveness of portable air purifiers (also called air cleaners or scrubbers) in the real world can be.
A global review of evidence, released this year, found portable air purifiers removed anywhere between 54 to 92 per cent of PM2.5 from indoor air.
Units fitted with high-efficiency particulate air (HEPA) filters are generally regarded as most effective at removing smoke particles, as well as pollen, dust, mould spores, bacteria and particles containing viruses.
But even the efficiency of the best HEPA filter purifiers varies widely. Many factors influence how well a portable air purifier works beyond the filter itself, not least how well sealed a home is.
Unfortunately, a lot of houses in Australia, especially older ones, are quite "leaky", which means more smoke can get in from the outside.
This constant influx of particulate pollution can decrease an air purifier's effectiveness. For instance, a study conducted in Australia found air purifiers with HEPA filters removed 30 to 74 per cent of PM2.5 from indoor air during a controlled burn.
A similar study in the US, where there are more tightly sealed homes, showed HEPA filter effectiveness ranged from 50 to 77 per cent.
Interestingly, although HEPA filters are regarded as the "gold standard", there is growing evidence that DIY air purifiers can be useful, although they haven't been fully evaluated.
If you do have a leaky house, do your best to seal gaps where you can.
But keep an eye on the air quality outside too, so you can flush the house with fresh air once the wind blows away the smoke outside. If you don't, the air quality inside could end up being worse than outside.
You can use apps such as AirRater to check the outdoor air quality, and most air purifiers will have a sensor to tell you what the air quality is inside — if you can't smell the smoke yourself.
While some air purifiers cost more than $1,000, you don't need to pay that much for a decent product — and you might get away with spending just a few hundred.
Still, even a small outlay eats into the household budget, especially once you add the cost of replacement filters.
So think about specific rooms in your house you need to keep clean. These are likely to be the ones where you or your family spend a lot of time.
The bigger the air purifier, the more expensive it usually is and the more noise it could make. Check the product information for how loud it is, in decibels (dB), when it's running.
As a guide, a whisper is about 30dB, a conversation is about 60dB, and a loud engine is around 95dB.
You might decide, for example, you want a larger unit for the lounge room where the kids can play in the day, and a smaller quieter one for the bedroom at night.
Research on using air purifiers to deal with smoke from wood heaters inside the house is relatively scarce.
Clean air groups have called for the introduction of tighter standards for wood heaters and low-emission technologies to reduce pollution.
But in the meantime, if you have a wood heater some tips to reduce, but not eliminate, smoke include:
Thanks to Sharon Campbell, Menzies Institute for Medical Research, University of Tasmania; Donna Green, University of New South Wales and Centre for Safe Air investigators; Amanda Wheeler, CSIRO Environment; Lidia Morawska, Queensland University of Technology; and Geoff Morgan, University of Sydney.
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