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How to Choose an Air Purifier to remove Traffic Pollutants

Although most basic hepa air purifiers can remove at least some of the pollution that come from traffic , only a few are specially designed to do a bona fide job. The main thing to bear in mind when choosing the best air purifier to deal with airborne traffic combustion pollutants is that the contamination is split into two parts, but most air purifiers only deal with one of these parts.

Firstly, there are the exhaust 'particulates'. Imagine these as tiny specks of soot that are so small you can not actually see them individually. Worry not however, many of these particles can be trapped inside the HEPA filter found in most common air purifiers these days (see more about this below). The second (and most difficult to filter out) type of pollutants from traffic pollution however are gases, such as Nitrogen Dioxide (No2) and Carbon Dioxide (Co2). Because these are gases and NOT particulates they cannot be trapped in a HEPA filter. Gases are so small they fly straight through any HEPA filter completely unimpeded.

In order to be certain of also removing the gases you need to ensure that whichever air purifier you choose has not just got a HEPA filter inside but also an activated carbon filter. Activated carbon is the only type of filter that can adsorb the harmful gases and keep them locked away. Beware however, not ALL carbon filters are the same. Many basic air purifiers which claim to have these filters inside only have an almost insignificant amount of activated carbon inside and therefore naturally only provide a token gesture of protection.

What size are the most common particles from traffic pollution?
Diesel exhausts are one of the most prolific sources of outdoor pollution. The particles sizes range from around 1 - 10 microns, with the largest volume being in the range up to 2.5 microns. Hence, you will often see traffic pollution referred to as 'PM2.5' in the media and on marketing material relating to air purifiers. PM2.5 ( particles with a diameter of 2.5 microns or less) are also released into our air from such common sources as log burners, coal fires, and industrial processes. These particulates are the ones you are often able to see as 'haze' or 'smog' in big cities around the world.

Can an air purifier actually filter out particles that small?
This is a key question! In short, yes, but some are more effective than others. The most common type of air purifier used to remove pollutants from traffic and other PM2.5 sources is one with a Hepa filter inside (Hepa stands for High Efficiency Particulate Arrestor). The grey area however is that there are different ‘grades’ of Hepa filter. Although most of them are perfectly capable of filtering out particles as small as 0.3 microns which more than covers the troublesome PM2.5 category, some hepa filters are actually able to remove particles as small as 0.1 micron, meaning they remove much more of the even more hazardous pollutants missed by standard ones.

So, any air purifier with a Hepa filter will effectively remove traffic pollution?
Not quite. The hepa filter is only part of the equation. As we mentioned at the beginning of the article, the other big issue is removing the accompanying gases. Remember, the hepa filter does not deal with the gases, it only removes particles (PM2.5).

So how do you remove the accompanying gaseous pollutants ( Co2, Nox, So2 )
These harmful (usually odourless) gases need to be adsorbed as they come through the filter and before they re-enter the room. The only thing that will effectively do that is an activated carbon filter placed directly behind the Hepa filter. The type and size of the carbon is directly related to just how much of the pollution is adsorbed. Expert air purifiers can contain around a kilo or more of genuine activated carbon. Most cheap and cheerful ( and some not so cheap ) units found in our High Street stores or an Amazon contain just a token gesture of carbon, often just a lightweight, thin piece of sponge which has been impregnated with a mere dusting of carbon. This token amount is all that is required for the manufacturer to claim that their machine 'contains' carbon and therefore removes gaseous pollutants. The reality is however that only a miniscule amount of pollutants could ever be adsorbed by such a tiny amount of carbon and therefore their benefit is virtually negligible. Always try to choose a machine with actual activated carbon granules in the filter as opposed to a simple carbon impregnated sponge.

Finally, Room Size - (always read the small print).
This is one of the most common mistakes people make when it comes to choosing an air purifier. We have already explained that your purifier needs to contain both a good quality Hepa filter and also a decent activated carbon filter. However, those two elements alone are rendered pointless unless the fan inside the purifier is powerful enough to drag all (not just some!) of the polluted air in your room size through those two filters frequently enough for it to stay ahead of the pollution. 

 Aim for around five air changes an hour to get the best results. Manufacturers will usually indicate what size room their equipment is designed to work in, but this is one of the areas where it is easiest to be misled as they rarely tell you how many air changes an hour they are providing. For instance if an air purifier claims to be 'Ideal for rooms up to 40m2' , how do you know how many times an hour it is actually purifying all the air in that room size? The answer is you don't. However, if the manufacturer actually specifies this 'Provides 5 complete air changes an hour in rooms up to 40m2'. That tells us that this particular machine will purify every bit of air in our 40m2 room at least once every twelve minutes which is excellent. 

One example of this is Bionaire  who manufacture a popular entry level air purifier called the BAP1700 and advertise it as being suitable for use in rooms up to 40m2. This makes it seem like a great option for larger rooms. However, Bionaire do not state how long it will take the unit to actually filter all of the air in that room. A quick check of the air volume in the fine print enables us to work out that used in a room of 40m2 that unit will actually take thirty eight minutes to clean all the air once. Therefore it provides around only 1.6 air changes per hour which is not that great. It would however be a good machine for rooms up to around 15m2 where it would indeed provide around five air changes per hour.

Now compare that to a specialist air purifier like the 205 Slim Smokestop from Swedish experts Blueair. Their 205 is only advertised as being ideal for rooms up to 26m2, however because Blueair design all their units to provide a generous 5 air changes per hour, even putting it in the same large 40m2 room as the Bionaire would see it clean the air around 58% quicker than the supposedly more powerful machine.

So the message is, take the advertised room size with a pinch of salt, always ask how many actual air changes the unit will provide in your room – regardless of what is claimed on the box.
In summary then, to remove traffic pollution as comprehensively as possible choose a machine with the following criteria:

1. A good quality Hepa filter ( filtering to 0.1 micron if possible, but filtering to not 0.3 will still suffice )
2. An activated carbon filter ( Activated granular carbon not impregnated sponge).
3. Enough air volume to change the air around 5 times every hour in your room size.

For more information feel free to call us directly on 01207 507444, we do love to talk to our customers. In the meantime to view a selection of specially chosen air purifiers with all of the correct criteria mentioned above click here:

View the best air purifiers for removal of traffic pollution