Almost any place away from cities. I've got AQI <30, down to 5 or so, almost the whole year for example. (Apart from during an occasional bushfire)

Cities and industrial sites are the exception with bad air quality, not the default.

To be fair, "cities are the exception with bad air" neglects the reality that cities are the default for where people live. So the "air near people" is generally city air.

That said, I live in SF and my AQI is usually <50. Not as great as 5, but we sometimes get down to single-digits. Cities don't have to have bad air.

That depends, really. If you have an inversion layer going, you can get pretty poor air quality in lots of locations because people use crappy wood-fired stoves that produce a lot of particulates.

That does happen occasionally where I live, last night for example. But it's only a single digit number of days out of a year. The rest of the time air quality is very good.

The filtration is icing on the cake. You want an HRV or ERV in any location where you want heating or cooling for any non-negligible portion of the year so that you can have energy efficient fresh air.

I live in small town on NZ coast. Air is very clean. Mosquitos and neighbor's wood burners don't care. ERV is top 3 item in my house (other than induction cooktop and Japanese toilets).

(ok nodding to ERV and induction cooktops, but what is a japanese toilet and why are they better?

Haven't heard of them from grand-designs and those kind of youtubers)

- auto lid open and close - heated seat - water jet to clean buttocks with subsequent warm air blower - relaxing music speaker (to drown out your defecating sounds and get you started)

just some ideas what some (public)/most (nice home) japanese toilets offer, which might be hard to come by anywhere else

> hard to come by

They are $200-300 online. Not Toto quality, but plenty good enough.

This is the first I've heard of auto lid open and close!

Only available on expensive models. Seat heating and washing are the key features.

The ERV itself is for removing CO2, not dust. Especially in cold climates where houses are built to be as air-tight as possible, these are a necessity. Even if you lived in a forest cabin you'd want filters to prevent too much dust and pollen. I've got a dual filter on mine, HEPA and activated carbon filter. The HEPA filter removes dust and pollen. I've found that if I don't use the carbon filter I get higher than recommended levels of NOx.

You can add a filter to the TW4, there is an adapter/kit. It's a hepa filter from a car cabin filter system design. You rarely need both anti pollen and also ERV, so you would take the heat exchanger out and just use the filter and fan. As for dust, I recommend a good pc fan filter based appliance, not putting the filter in the flow path of the ERV.

Most people here just open a window when they want to air out a room. That said, that does waste a lot of energy when heating/cooling your home on cold/hot days, so ERVs and HRVs are used to get the clean air in without exchanging heat with the outside world too much. They're quite cheap and effective compared to just running normal ventilation.