It is a known fact in Singapore that air purifiers and N95 masks will be in hot demand during the haze season.

We may, however, be less aware of the air pollution in other countries which may be caused by other factors. Case in point: I found myself preparing for a trip to Paris and learned that the city was having even worse air pollution than Beijing. And I was going to stay there for nearly a week, commuting by foot. And the hotel I would be staying in is along a main avenue that leads to the Arc de Triomphe, which means there’d be plenty of traffic exhaust.

Now, I’m not too sure whether the French hotels bother to purify their air when there’s serious pollution. I wasn’t going to risk spending a week inhaling polluted air, if I could help it. So I decided to look for mobile air purifiers. Then many questions popped into my head.

1) What sort of technology should I be looking for?

As a layperson I heard that HEPA filters were good while ionisers were not as reliable. Trawling through many websites and forums, it appeared that there were two camps: Those that swore by HEPA and filters, and those that didn’t. But HEPA only filters particle sizes of 0.3 microns (micrometres, or 1/1000th of a milligram). This would be fine if we were only looking to remove things like pet allergens, pollen, spores, dust mites, bacteria and fungi.

However, that led to the next (and more key question):

2) What am I actually trying to remove or purify, in Parisian air?

That led me to a detailed search where I found two key documents: A Guardian article on how Paris air pollution is caused by diesel vehicles, industry and agriculture. Given that I was living in the city centre and that two thirds of motorists use diesel vehicles in Paris due to some pretty poor policy decision-making, I searched for the particle size of diesel exhaust. And voila! This document from the University of Minnesota found that quite a lot of diesel exhaust particles were tiny – technically labelled as ‘nano’ as they were less than 0.05 microns in size. Some were even as small, or smaller, than 0.01.

And you know how people like to smoke in Paris (I expect this to happen even at healthcare conferences). Generally I have found that people still smoke in non-smoking rooms – you can tell from the smell. Tobacco smoke is 0.003 to 0.04 microns in size. Again, a HEPA air purifier will not remove this.

So, given that the most dangerous stuff would be the nano particles that your lungs can’t get rid of, I wanted an air purifier that could do more than HEPA. But that led to the next question:

3) What was effective? And portable?

This Youtube video by the Air Purifier Guy was useful, if slightly dated. I was also looking for something in a different context – I was more concerned about vehicle exhaust particles. But I took the point that ideally, a good air purifier should have multiple technologies. Companies also like to rip off consumers by making them buy overpriced filters (just as with people who buy cheap printers with expensive cartridges) and don’t give them sufficient information about each air purifier, to make a qualified decision.

So I kind of had an idea about what would be effective. Next, what was portable? Further internet searches led to a product which was the size of a modem but claims to carry the same photocatalytic air purifying technology as NASA, for growing fruits. It also had a UVC light and ion generator. It can remove particles up to 0.01 microns, which is what I was looking for. Larger particles will be collected in a washable filter. It can also be installed in your car.

A phone call to the local dealer led to me having a chat with the boss, who followed up quickly to check if stock was available. Since I needed to replace a component of this mobile air purifier every year, I wanted to ensure that this company was quick in following up with customers. I am hopeful about this device but still a shred of doubt remains, as the air purifier market is flooded with so many products and technologies – and jargon that confuses the casual shopper – that I will test it out first before adding a link to its product page. Still, it is very hard to prove whether the air purifier removes nano particles – unless I  bother (and can spend a few thousand dollars) to get laboratory equipment. I will just have to trust its scientific lab reports.

While doing my research, I also did not like how some brands claim to have HEPA and Nano filters, but in fact these ‘Nano’ filters do not filter out the truly ‘nano’ particles. To cite the wisdom of Wikipedia on particle sizes:

Particles are further classified according to diameter.[1] Coarse particles cover a range between 2,500 and 10,000 nanometers. Fine particles are sized between 100 and 2,500 nanometers. Ultrafine particles, or nanoparticles, are between 1 and 100 nanometers in size.

1 nanometer is 0.001 microns. 100 nanometers is 0.1 microns. As mentioned earlier, HEPA filters only address particles up to 0.3 microns and I am still trying to find a commercial website that states the particle size that their ‘nano’ technology is able to remove. These nano particles are so small, filter technology can’t block them.

4) Conclusion

If you’re searching for an air purifier, you need to be very clear what it is you’re trying to remove. Is it the haze which you can see (i.e. of larger particles)? Or a certain odour that won’t go away? A certain allergen? Or are you trying to avoid catching a bug from someone else near you? Then, find out the particle sizes that you need to remove, and an air purifier with the technologies that can do the job.

You also need to know the volume of the room where you intend to place your air purifier, so that you get one with the right capacity. Other things to consider are the cost of maintaining it and how noisy it can be (although again, these less desirable points are not going to be easily found on product web pages).

For me, being near the centre of Paris, it’s literally the finer things in life that matter the most.