As people start coming into hospitals with the coronavirus, the number of masks we go through with standard protocols goes up enormously. These masks are normally single-use, and you put on a new mask every time one is needed. Roughly, a hospital could increase its daily usage of masks 100x as they get their first few covid-19 patients, and then even more as the full force of the epidemic hits. This is a ton of stress on the supply chain, and not surprisingly suppliers haven't been able to ramp up. Running your factory around the clock and bringing on extra workers can help some, but when even doubling output would be impressive this is nowhere near enough.

There are many types of mask, but the two main ones in health care are surgical masks and N95 respirator masks:

• A surgical mask is primarily intended to protect others from the wearer by catching droplets, but provides limited protection to the wearer.

• A vented N95 mask protects the wearer against not just droplets but also airborne transmission.

• An unvented N95 mask protects both the wearer and others.

When I say "mask" below, I'm talking about N95 masks. We can get something to replace surgical masks, even if it's people sewing reusable cloth ones, but N95 production is bottlenecked on machines that can make good enough melt-blown fabric.

Luckily, health care is not the only field where people need respiratory protection. Industrial N95 masks are very widely used in construction, demolition, and other situations where there's moderately hazardous dust. These masks aren't rated as surgical N95 masks, and they're more likely to be vented, but their requirements are very similar and the government is now allowing them to be used.

As hospitals are unable to get resupplied with their regular masks, they're asking for donations from the community and industry. This makes a lot of sense: people and organizations that use masks generally keep extras, and medical use is now much more urgent.

On the other hand, donations of masks will not get us through this epidemic on their own: hospitals also need to make massive adjustments in how quickly they go through masks, and this is a hard adjustment. Reusing masks is moderately dangerous, but it's much less dangerous than the very likely prospect of later not having them at all. It looks like hospitals used masks at nearly their regular rate throughout February and in early March, even though the shortage goes back to late January. Reports of mask rationing are haphazard, and in the last couple days I've seen posts from health care workers saying they're using N95 masks:

• At their regular rate, but they're worried about running out.
• For aerosol-generating procedures on suspected patients only.
• One per day, only as needed.
• One indefinitely.
• Not at all, because there are no more.

Hospitals are not the kind of places that change policy quickly, and it seems they're mostly adjusting in response to their resupply orders not being filled. The CDC should be telling hospitals across the country that we're very short on masks and that we should be going through them as slowly as possible, but instead it's offering much weaker guidance that would be more appropriate to local shortages.

Since most of this change in behavior is happening in response to masks being unavailable or in very short supply, mask production is hard to ramp up, and we don't expect this to peak for at least a month, if you donate masks today I expect them to be used much more quickly than if you wait and donate them when things are worse. You don't want to wait too long, because at some point the shortage really will be over and the need will decrease, but I expect the need for masks to be much higher in two weeks than it is today.