The mask Tom's wearing in almost all the pics from various 'Mission: Impossible 7' sets is NOT recommended by the CDC ... and the agency actually warned against wearing any mask with valves like the one TC's rocking.

The mask, from Jupiter Gear, features a carbon filter and breathing valves ... and it retails for $14.99 a pop.

The manufacturer says the mask is meant to mostly be worn during physical activity, with the valves designed to "release hot, humid exhaled breath quickly." In other words, there's still a good chance respiratory droplets are escaping ... and that's dangerous with this airborne coronavirus."

In fact, the CDC spells it out, warning against masks with valves because ... "this type of mask may not prevent you from spreading COVID-19 to others. The hole in the material may allow your respiratory droplets to escape and reach others."

As you know ... Tom blew a gasket over film crew members breaking COVID protocols by standing too close together on set, and promised to fire them if it happens again.

Now, just as ineffective as Tom's mask -- and probably worse -- is what Tom's costar Hayley Atwell was seen doing on set in Rome. She's one of those folks who wear their mask under their nose which is also highly ineffective in stopping moisture droplets.