Usually yes. But it does depend on the air quality outside and how leaky the building is - in certain places with very poor air quality you'd need a very seriou

Usually yes. But it does depend on the air quality outside and how leaky the building is - in certain places with very poor air quality you'd need a very serious flow rate through your purifier to keep up unless you have a positive pressure system (where you filter the air coming in to the building instead of just recirculating through a filter), and keep it at a positive pressure to stop dirty air constantly leaking in (especially when running bathroom exhaust fans or kitchen rangehoods that vent outside). In fact there's an article on the very same blog as the original post about it - https://www.airgradient.com/blog/positive-pressure-systems-for-clean-air/ https://www.airgradient.com/blog/positive-pressure-systems-f... Of course the downside is larger load on air conditioning, at which point you can look at heat recovery or energy recovery ventilation systems. I'm quite keen to get one of those to get more fresh air in since I can't leave my front or most of the side windows of the house open at night (road gets too noisy in the morning).