The goal is to minimize the amount of contaminated particles (droplets, dust, etc) that you exhale and inhale. If you're carrying the mask reduces how many particles your colleagues come in contact with, if your colleagues are carrying it reduces the amount of contaminated particles the you inhale. When both of you are wearing masks then the likelihood of transmission decreases further.

The likelihood of inhaling is then dependent on how frequently you breath in, and vice versa. So what matters is not just who you have contact with, but rather how long the contact lasts. If you're in the office let's assume you have a 10 hour work day, with 1 hour for breaks where you're unmasked in an area with other unmasked people.

Let's do some very basic maths, using some very dumb assumptions

  * Wearing a mask reduces your likelihood of inhaling a contaminated particle by 50%

  * Wearing a mask reduces your likelihood of emission a particle by 50%

  * If you are in the vicinity of an unmasked carrier you have a 1% chance of inhaling it every hour

  * If a contaminated particle is inhaled you have a 50% chance of catching the virus

This obviously ignores things like proximity, immune strength, real world transmissability, etc. The number are chosen entirely for the ease of maths.

Using these assumptions, if you spend 10 unmasked hours with a contagious coworker then your likelihood of inhaling a particle in 10 hours is something like

  1 - (0.99 ^^ 10) = ~0.096

Or 9.6%, if you do inhale a particle you catch the disease 50% of the time, so you have a 5% of contracting the virus in ten unmasked hours.

If you are both masked all the time, then the likelihood of exhalation drops 50% and your inhalation drops 50%, so your likelihood of inhalation drops from 1% to 0.25%, the above math becomes

  1 - (0.9975 ^^ 10) = ~0.025

Or 2.5%, the above maths again gives you 1.5% likelihood contraction.

If we go for 9 hours mask on, and 1 hour mask off you get

  1 - (0.9975 ^^ 9 * 0.99) = ~0.032

Or 3.2%.

Now it's important to note that I'm pulling these percentages out of the air. I would guess that masks are more effective, likelihood of inhaling a contaminated particle in the vicinity of a carrier is higher, but your likelihood of catching it when you do inhale is lower (this is also the number impacted by vaccination).

The important thing is that the only thing we can control directly is mask wearing, so all other figures can change and the end result is always that masking lowers risk. If you must remove your mask (e.g. for eating) the less time you are unmasked the better.

This applies all the time, for any airborne disease, not just covid.