I have an unconventional recommendation: Start with measuring.

The hard part about collecting invisible dust is that it is invisible. I bought little meter from Aliexpress that has a Plantower PMS5003 sensor in it. When I bought it USD$33.33 delivered to my door. They have a variety of names, but it looks like this.

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Measuring particles is super difficult, so I don't expect this little piece of science fair equipment to be precise, but it does correlate with the air quality readings published by the government and I have been able to tell which activities are the worst.

My shop is about 3m x 6m with a garage door at one end and a person door at the other end.

The baseline depends on the air outside, but let's say we start with a PM2.5 of 15. If I have everything closed and I sweep the floor, the value will shoot up to 50 or more. Likewise, if my son turned a pen on the lathe with no dust collection, it would go to over 100 easily.

If I open just the garage door and blow in a fan, just as Bob said, it doesn't help the numbers.

However, if I open the garage door and the person door and setup a cross breeze, then the numbers settle down quickly.

It sounds like you can work with the doors open. Is there any way to install a centrifugal duct fan with 2-3 openings at the far side of your shop and then blow the air outside? That will make a big difference. Ideally you would have 20 air changes an hour.

The other thing I learned from the meter is that the fine dust says around for weeks. My son used a 1x42" belt sander with no dust collection and there was fine dust everywhere. I scolded him and then used a wet rag to clean up what I could see. For weeks after that, the particle counts were higher than I expected. So I opened everything up, put on a mask, and used an electric leaf blower. I couldn't believe the cloud of dust. After I did that, the numbers stayed lower. That fine dust had coated all the surfaces and even with my how powerful dust collector, the numbers stayed high. So cleaning helps.

I used to have 1.5hp unit with a 10" impeller and a 5 micron bag. I bought that collector about 20 years ago. It was all I could afford at the time. I don't really trust the numbers, but with good cross ventilation, the particle counter didn't seem to elevate that much when I ran the dust collector and the planer or bandsaw. If I left it running, the numbers would slowly drop.

So what I'm suggesting is a) measure what you have now b) get ventilation if at all possible c) just do the best you can on the dust collector. If you are able to measure, then it removes some of the stress from this situation.

(Now's the time when everybody else tells me I don't know what I'm talking about.)

Mark