When buying N95 masks there are many options on the market. One choice you will come across is to choose a standard N95 respirator without a valve, or to get a mask with a valve.
The valve in facemasks are there to make the mask more comfortable for you, especially when wearing the mask for longer periods of time. When you exhale, the valve opens up and lets the hot air through the valve instead of having to force the air through the mask filter. This makes it easier to breath and also keeps you cooler. It also helps the mask not get damp / soiled as it lets the hot and humid exhalation air out quickly and directly.
Masks with valves will still protect your inhaled breaths as when you breath in, the valve is closed and the mask will filter as a standard mask would do.
This is what 3M says about their 3M Cool Flow valves:
“The Cool Flow valve’s efficiency in keeping breathing cool and comfortable has been demonstrated through testing*: the atmosphere inside a respirator with a Cool Flow valve is on average 7.5°F cooler than the similar product without the valve. This makes Cool Flow valve respirators ideal for long periods of wear, especially where conditions are hot, humid or physically demanding. *Testing conducted in a 3M laboratory. Testing protocol, data generation and conclusions were reviewed and approved by an expert from The University of Minnesota”
It is also important to note when you should not use a valved mask. If you are in a sterile healthcare environment you SHOULD NOT wear a valved mask. The reason is that the mask will not filter out contaminants from your exhaled breath. This will contaminate the environment around the wearer. By the same token, for containment purposes for example, if someone is sick with something transmissible or contagious they should wear a standard mask, NOT a valved mask.
There you have it, now you know when to choose a standard N95 mask and when to choose a valved N95 facemask.