Here in New Zealand, what they call 'HRV systems' actually draw air in from the attic (more on that subject another time!). There is however at least one system I've found available here now that is a 'real' HRV that draws in "fresh outside air". Problem is, 7 or 8 months of the year in the city where I live the outside air is far from 'fresh' - it's one of the most polluted cities in the world from logburner/open fireplace smog. I can't find any information on these systems that even mentions this concern, much less discusses if or how these systems (can) deal with air pollution. Anyone know if air pollution, especially dangerous particulates from logburner smoke, can be filtered to an 'acceptable' degree through these systems?

Perhaps I should frame my question a bit differently (the way I've put it above sounds like I'm only interested from my own somewhat unique situation)?

Unless these HRV systems are intended for use only in rural or isolated environments where the outside air is relatively 'clean' ("fresh" in my book implies "clean"), then surely how these systems deal with smog/air pollution, which certainly exists in most urban areas, would be a question that EVERYONE contemplating installing them would want answered? Have any others of you felt cause to pause when reading the words 'fresh outside air' in the blurb for HRV systems?

This is actually a question that I thought would apply to many of us - if I was in ANY urban or semi-urban environment, logburner smoke or not, I'd be asking: if the outside 'fresh' air that the system is drawing from is polluted, a) What effect does this have on the quality of my indoor air when using this system, and/or b) Does this system protect me from harmful external-air pollutants, and if so 1) how, and 2) how well?

(Tom: Thanks for the suggestions! Unfortunately a) no, can't move out just yet, but definitely working on it and b) a 'clean heat project' has been running for 5 years now in the city with subsidies offered for switching to 'cleaner' heating systems. I honestly can't tell the difference in the air now from 5 years ago, the clean air standards are breached here same number of days now as then (approx 30 days a year for the city, but if you have even a few neighbours who heat their homes and water with wood, that number might as well be tripled or quadrupled for how you personally are affected). Much of NZ is hydro-powered and this year as most years we are being threatened with winter power cuts because of poor rainfall -the smog problem will not go away until NZ gov't gets its act together to assure a reliable winter power supply.)

The quality of the external air doesn't really affect the point of the HRV system, which is to ventilate your house in an energy efficient manner. After all, you need the same amount of ventilation irrespective of whether it's supplied mechanically or by opening a window.

In fact, HRV is more likely to be specified in an urban environment where windows must be kept closed because of noise issues. All systems will have inlet air filters, so they will improve the quality of the air. If you are worried about particulate pollution (generically known as PM10 and PM25) then you can fit higher specification filters (EU7 standard, for example). These will increase the energy required by the the fans, and will need changing regularly.