Gas mask vs natural toxins?

If by toxins you mean the massive ejections of sulfur dioxide (along with hydrogen, hydrogen sulfide, hydrogen chloride, carbon monoxide, and carbon dioxide) that follows volcanic eruptions, then yes. That's what gas masks are for, toxic gases except for carbon monoxide which even as a toxic gas should be the least of your concern. Your filters must be new, unopened, and unexpired for maximum efficacy i.e. not surplus filters. These new filters are found almost exclusively online or at a industrial supply store like Grainger. The NATO standard are 40mm threaded canister filters, so almost any 40mm filters can have the NATO label attached to it.

CBRN and NBC filters rated for acid gases will do. Here's some examples by Avon, 3M, MSA, Mestel, and SEA, note where it says sulfur dioxide and hydrogen sulfide as well as hydrogen chloride:

http://www.avon-protection.com/Downloads/2011_us/C...

http://multimedia.3m.com/mws/mediawebserver?mwsId=...

http://media.msanet.com/NA/USA/DomesticPreparednes...

http://www.mestelsafety.com/products_filters.html

http://www.sea.com.au/html/products/negpress/datas...

I don't know whether it can last an hour under Yellowstone, maybe but it might not. It depends on the level of contamination by concentration. That's why you ought to get several filters or however many you can afford for changing. Something like a self contained breathing apparatus for use with gas masks guarantees an hour of breathing time in the oxygen tank but SCBAs are expensive and cumbersome. Have your gear within reach, and start evacuating if there's a warning or if the caldera goes off. That means have a sturdy, reliable car with gas in it ready to get out of there as far away as possible or any signs things are becoming unstable and use the backroads or offroad if possible or if needed in case of any ensuing chaos and if major highways get backed up. I also can't predict how severe the ash storm will be but it could be enough to bury everything around ground zero meaning buildings will collapse from the weight of ash building up and travel would be hindered or impossible.

The HEPA filters used with gas masks should filter out all the ash and glass-like particles but it could also plug up the filter fast and make breathing more difficult. It's havoc to your car and its filters too.

Also sulfur dioxide can create acid rain. If you're caught in the open (which you ought not to) you need a suit like a charcoal-lined suit, coveralls from Home Depot like something made of Tyvek, or even a PVC rain poncho.

And be sure your gas mask is not an obsolete model, in which most Cold War era or older masks are.

Just a follow up on MC's reply - You never stay in a hazardous area. Always run away from it. It's almost impossible to buy graded filters, if possible, these are some really expensive filters.

Also, Gas Masks from the 70's are still good, so long as you stay away from anything that's heavy.