I see these come 2 adapters in a pack.........gas mask has a right and left side port.  Do both adapters need to be used when using the p100 filters or can you use one adapter and p100

usually the mask only has ONE filter fitted (M-40 or MCU2P) so you would only need ONE adaptor.

the masks have the OPTION to fit the filter on either side to accommodate Lefties or Righties when mounting weapon to their shoulders.

Filter goes on one side, and a blank "plug" (or Voicemitter) goes on other side to block airflow in.

BIGGER_HAMMER

Not exactly.

40mm filters are designed to be used individually. The 3m twist lock filters are designed to be used in pairs. Even just the filter port you'll notice a little restriction due to the inside diameter. The big pancake p100 filters have a lot of surface area so could probably get by but I wouldn't do a single filter with any other type.

Hmmm I need a tie breaker lol.   I thought it would be ok to just use one 3m filter .  Now I’m second guessing

Filters come in pairs, the adapter comes in pairs.
Trying both and see what works for you.
If you have a strong respiratory system, and aren't doing anything strenuous you likely only need one.

This.

Tie Breaker - I concede the point.

I try not to alter my masks so I can't just run spin off the P100 & screw on the Mil-Spec 40mm filter if it unexpectedly "Party Time"  .  I'm not expecting Phosgene (or VX ), but think it very likely to encounter lots of Riot Gases & Pepper if things get "Sporty" in another "Urban Insurrection".

If you wanted to set up a Mask pretty much just for P100, then using BOTH filters as intake will allow more P100 filtered air in with less restriction for easier breathing if doing "high air needs" physical stuff.

For what is worth, the US Military canisters (& NATO Mil-Spec standard calls for the NATO 40mm filters to be effective against BIO-WAR agents.

I keep a M-17A2 Mask with fresh filters in a sealed storage bag for COVID - Disease - BIO stuff because it has the dual filters (cheek) while still being effective against ABC / NBC / CBRN.   I HATED the M-17 mask (those fooking cheek filters with the tiny post & slots! ) which make it impossible to change filters IN an actual ABC / NBC / CBRN environment without taking the Fooking mask OFF your head)

Oh Well!   Good Luck O.P.!

BIGGER_HAMMER

There is no such thing as a "fresh" filter for an M17. All filters for M17s are long-ago expired (by decades) and dangerous to use due to toxic materials in their construction. IIRC the last production filters expired in the late 80s.

No filter for an M17 is "CBRN" effective, as military filters do not protect against some very common industrial chemical hazards (ammonia category) even when they were new.

All M17 masks are decades past their retire date and their effectiveness is questionable at best. All it takes for any mask to lose all effectiveness is for the exhalation valve disk to harden up and lose the seal against the mask

As I post over and over, cannot strongly recommend enough to dispose of ancient military surplus crap and get a current industrial respirator with in-date cartridges. Multigas plus P100 covers almost everything that an air-purifying respirator can be used against, CBRN rated gets you longer run time in exchange for larger, heavier and harder to breathe through. I usually suggest just the regular filters with spares.

There are decent 3m and Mestel full face options that take modern 40mm canisters available for under 200 bucks, sometimes cheaper. No need for an ancient mask without modern filter options.

I went with this.
They are available on eBay.

I agree with avoiding old surplus filters and even .mil masks.  They suck and are not comfortable - filters are heavy.

I've got a mil-spec mask and a 3m FX with the p100/magenta olive filters - basically the stop everything filters.  3m FX is super comfy and although it sucks to shoot a rifle with... the field of view is awesome.

3m FX

Hopefully its abundantly obvious to all but just in case its not...

Gas masks only filter intake air, not exhaling air...does nothing about exhausted air from lungs.

Yes it is obvious, but who cares about the exhaust. Why should I care to filter it.
My greater concern is a mask can't help in an oxygen depleted environment.

Just put a surgical mask over your gasmask.