I have a “decent” ERV. The manufacturer claims that it’s great against air pollution, wildfire smoke, etc, and those claims do not come with numbers. Empirically (as measured by an actual particle counter), it’s crap. It does have real numbers for ventilation rate, and it works great for ventilation. This is a high end ERV, and I see no evidence that any other model is better. And it can’t be: a good, energy efficient filter is large, and all the manufacturers want their devices to be small.
My solution: put a great honking filter in front of it. This increases the lifetime of its crap internal supply filter to effectively infinity (or I could remove it), adds basically no resistance, and reduces supply particulates to effectively zero.
“Great honking” here means a 24"x24"x12" “nominal” HEPA filter with a MERV 8 prefilter. The prefilter costs basically nothing. I expect the HEPA filter to last for years. I would have preferred a not-quite-HEPA 99.5% or 99.9% filter instead, but those are surprisingly hard to buy. Big HEPA filters are easy to buy. Two big MERV 16 filters in series would do the trick, too.
(The filter efficiency math is entirely different for a ventilation system than for a recirculating filter. For recirculation, MERV 13 is fine and MERV 16 is a bit better. For once—and-done ventilation, you want to reduce outdoor crap to your preferred levels in a single pass, and MERV 13 is not even close to good enough in places with pollution or smoke issues.)
Coversely, a poorly filtered fresh air system is fine for reducing air contaminants from indoors but will actively fight against a standalone air purifier if your goal is to reduce outdoor pollutants.
Probably depends more on the air changes per hour. You'd have to compare running a filter in your room/house and the air changes it'd go through in an hour compared to your ERV/HRV. My bet is that an ERV is better if they're the same. But, it is much easier to add capacity to individual rooms with a standalone air filter than it is to change your ERV to be higher output.
I think using a HEPA air purifier can still be beneficial. ERV and HRV typically have standard filters that may not capture very fine particles, allergens, or airborne pathogens as effectively as HEPA filters do.
