Reddit mentions of 3M 2091 P100 Particulate Filter, One Pair Per Pack

And not just any dust mask! Get a half-mask respirator with P100 filters. So much more particulate can get through that old paper dust mask than you'd ever think. P100's are the highest level of filtration out there, and still super cheap.
https://www.amazon.com/3M-2091-P100-Particulate-Filter/dp/B002KFFY9A/ref=sr_1_3?ie=UTF8&qid=1499727326&sr=8-3&keywords=p100

Mask:
https://smile.amazon.com/gp/product/B008MCUULW/ref=oh_aui_search_detailpage?ie=UTF8&psc=1

Filters:
https://smile.amazon.com/gp/product/B002KFFY9A/ref=oh_aui_detailpage_o00_s02?ie=UTF8&th=1

$25 + 6 and you've got it covered.

Even the same exact pile of powder will have scoops varying as much as double/half due to how you scoop it. I use the Gemini-20 which is pretty reliable as long as you use it properly and weigh in the neighborhood of 20-30mg+ and dilute volumetrically; you're not going to get any more reliable unless you chip in at least $2-300. Controlling air currents is still pretty important and if you're really particular, you can grab particulate filters but meh, I've gotten comfortable with it. I actually trust my vendors a FUCK TON more than I do street drugs, both in purity and a feedback system of other people that know what they're doing, who sometimes send it for GC/MS testing for everybody.

I use this with these. they work wonderfully for me and seal over my beard.

If you're going to use a paper dust-mask, you need one with a "down-firing exhaust". But, I'm going to go ahead and suggest you upgrade to a respirator with filters. They're more comfortable, get a better seal against your face, have replaceable filters, and universally have forward or down-firing exhausts. By the time you've used a handfull of dust masks, the respirator is cheaper too.

My go-to respirator is the 3M 6000 series - they come in small, medium, and large - but you want a medium unless you have really weird facial proportions. You'll need to pair it with some filters, they're available in P95 and P100. Personally I find the P95 filters are more than good enough for me.

There's a whole 3M respirator ecosystem though - I'm going to shamelessly link to a nice-long comment on selecting the respirator components and the resulting discussion.

edit: My wife wears a 3M 6100 (the small) with prescription glasses without issue and I wear a 6200 with generic poly-carbonate safety glasses and find it quite comfortable.

Not all eye protection is the same. I use the 3M Lexa Splash GoggleGear. Those were recommended by 3M for simultaneous use with a respirator. Regular glasses don't have any protection around the eyes, so when wearing a respirator, the dust is funneled right into my eyes under the lenses. Larger cheap $4 splash goggles were too large to work with the respirator.

Also, that respirator I linked to is just the reusable part. He'll need filters. For only dust, a pair of cheap P100 filters will work. As soon as he starts painting, using chemical finishes, anything with a vapor, he'll need something more versatile that absorbs the chemicals as well as dust.

If you're going to get a respirator, start the comfy 7500 series and get a P100 filter. Why filter 95% of dangerous particles when you can filter 100%?

I just picked up one of these too and it works great. I like the fact that the exhaust vent is down-firing so it doesn't fog up my safety glasses. The only problem I had was that I found it nearly impossible to figure out which set of filters/attachments I needed from the various pages I read online. I eventually, after two rounds of purchasing, ended up with:

• 3m 7500 series mask
  
• 6000 series cartridge filters
  
• 502 filter adapters
  
• 2000 series big pink P100 prefilters
  
  Having read all the instructions it seems like there is a way to skip #2 and #3 and just connect up the big pink P100 filters to the respirator, maybe with another adaptor in between, but I'm happy with what I ended up with.