Why does my mask smell?

 My mask smells, why?

“I get this question a lot from my patients, and it’s definitely an important one. My response is: How long have you been using the face mask?” says Darien Sutton, MD, an emergency physician at Bellevue Hospital in New York City.
The reason he asks is because an “off” smell is a telltale sign that it’s time to toss a disposable mask, or wash a reusable one.  Particularly the cotton and linen that hold moisture droplets smell faster.  BYOM's polypropylene filter does does not smell for a long time because it does not hold moisture.  When it does it is time to wash.   
When you open up a new mask there may be another chemical smell - when you open a BYOM - there is no smell we only use water based dyes and no chemical treatments in our fabrics.
“When you’re using a surgical mask, you’re breathing on it and your respiratory particles land on the mask. The bacteria in those particles can then grow and cause an odour you find distasteful,” Dr. Sutton explains.
The other main reason it can smell if a new or newly washed mask - is your breath.  "Your breath was probably already a tad putrid when you put that mask on, explained Dr. Mark S. Wolff, a dentist and dean of the University of Pennsylvania’s School of Dental Medicine, who has spent many years researching all the reasons our breath can sometimes be stinky. The problem, Wolff said, is that we don’t know it because we’ve just recently began exhaling into a covering that hovers over our noses all the time. “It’s as if we cupped our hands in front of our face to test our breath,” Wolff said. “But it’s there all the time.”