30 minutes? Definitely not.

More than once per day? It's complicated. There are two designs:

- Electrostatic masks work well so long as the mask remains charged (those are the 3M N95 ones). Efficiency drops as they become moist, especially. However, for good ones, even after extended use, it's still surprisingly high.

- Nano masks basically work forever. The initial efficiency is lower than electrostatic, but remains pretty good for longer than I'd want to use one.

For real-world use, I really prefer nano. Unless you're much more careful than I am, enough air leaks around the mask that I'm not sure there's a big difference between 99%, 95%, or even 80% filtration.

And the numbers are extremely conservative too -- they look at filtration around a few hundred nanometers, which is the worst-case. Most diseases come in on much larger droplets.

One thing which tends to be under-emphasized is that it's not binary. Viral load is important. A single virus particle probably won't cause an infection (it probably won't infect a cell, and if it does, your innate immune system will probably flush it). If it does cause an infection, your adaptive immune system has a lot of time to react, so it probably won't be a bad one.

This is even more true for pollution and allergens.

For my purposes, I find nano masks to be the right bang-for-the-buck. They're much more breathable too. And they handle environmentals like rain or sweat better.

The Asians are right; living in a big city, I've felt better since 2020, with masks, air quality monitors, and HEPA filters everywhere. I get sick less often, have milder seasonal allergies, and just generally do better.

On a related note, other aspects of air quality matter too: CO2 levels, humidity, etc.

No. I've been using N95s since the start of the pandemic and have spent a fair amount of time learning about them, e.g., reading the written material that comes with the box of masks, and I've never seen that kind of recommendation. The written materials say things like, "throw the mask away as soon as it becomes stained or discolored."

I use each of my masks for probably at least 10 hours before throwing it away, and I've never gotten covid except once (in Jan or Feb 2020) before I started wearing masks (in fact, before anyone in my country -- the US -- knew about covid).

(I have to write "probably" because I don't actually keep written records of how long I have each mask on. I rarely have one on for more than an hour at a time.)

I've used mainly 3M 8511 masks.

The most important distinction between mask models is that some models have valves / vents and some do not.

Hospitals use ones without valves. The lack of a valve means that the part of the mask that filters the air gets a lot more damp than it would if a value were present (because exhaled air is damp) which might drastically reduce how long the mask will last, so that might be why some hospitals instruct their employees to use a new mask for each new patient contact.