Sadly, the majority of homes and businesses in the UK are exposed to levels of harmful Particulate Matter above W.H.O safe limits. PM2.5 microparticles are the most problematic because they’re too small for the lung to deal with. Rubber dust from worn car tyres and soot from combustion engines are among the worst, although mould spores and pollen can both be below 2.5 micrometers. Fortunately, some MVHR systems filter these microparticles to keep them out of the building. But many properties are also exposed to noxious gasses too, particularly those near busy roads or in city centres.
Besides harming the environment, NOx gasses exacerbate symptoms for those with heart or lung conditions, and form Ozone that triggers inflammation of the respiratory tract, eyes, nose and throat as well as asthma attacks. And the worst part is that it’s very difficult to separate harmful gas from the fresh air you need for ventilation.
Fortunately, NOx gas filters are now available for domestic MVHR, and for the price of a TV (purchase cost) plus a Netflix account (servicing cost) you can stop enough noxious gas to keep your air within safe limits. However, you can’t simply bolt one on to any old system. They’re up to a metre long and very heavy so you can’t dangle one in fresh air. It needs fixing to the building structure somewhere it can be serviced easily. Gas filters go after the particle filters so they don’t get clogged with leaves and dust, normally on the main air supply pipe after the MVHR unit, and for an existing system that pipe is normally hidden. But the biggest challenge is handling the considerable air resistance they add to the whole system, sometimes as much as the rest of the ducting combined.
A decent active carbon filter can add 100 pascals of resistance, or system pressure. If the rest of the system is not designed to accommodate this extra load, you’ll end up thrashing the machine, wearing out the motors, under-ventilating, and exceeding the noise and energy regulations. To handle this extra pressure quietly and efficiently you may need a more powerful machine, more ducting, and bigger pipes. In other words, if you want NOx filtration it should be designed from the outset.