Here he claims masks (general) do not protect you. But they do protect healthcare workers. It amazes me that he was willing to put two such contradictory claims in the same tweet. It was just such a transparent lie. Given the context and implication, I think it is referring to "medical" grade masks including N95, surgical masks, etc.

Another part of the confusion is that early on (six weeks ago) the assumption was that the virus spread like the flu, i.e. via droplets from coughing and sneezing. Masks alone (without face shields) are of little benefit for droplet-spread disease because they don't protect your eyes. Now we know the virus spreads via aerosols which are generated by talking or even breathing. Cloth masks do indeed help for aerosols by trapping virus particles electrostatically and by increasing humidity, both of which keep the virus out of your respiratory tract. Or so the latest thinking goes.

Hanlon's Razor applies, but this virus is also an insidious bastard that behaves very differently from what public health experts had expected.

First of all, there are very different rationales for healthcare workers and the general public to wear masks. Healthcare workers know they will be exposed, so the protocols must be damn near perfect. The general population does not know if they’ll be exposed, they’re trying to work in aggregate to put a drag on the viruses’ spread; even partially effective mask wearing slows that down. The general population can also practice social distancing, while healthcare workers must spend hours in close personal contact with positive patients by the nature of their jobs, vastly increasing their risk.

Second, this ignores the fact that mass mask usage prevents you from getting someone else sick. This is in fact exactly why surgeons wear surgical masks. Since we’ve known for months that this disease has asymptomatic transmission, the public health rationale for even cloth mask usage was obvious from the very beginning.

Without the rest of the kit, and without careful adherence to correct donning and doffing, there are at best marginal benefits to mask wearing.

There's some concern that those marginal benefits are eliminated if people wearing masks go on to engage in risky behaviours. These behaviours include increased touching of their face (often without immediate handwashing afterwards), reductions in distance between people, increase in frequency of leaving the home, etc etc.

PPE is designed to mitigate a completely different level and type of risk. PPE is designed for medical professionals who by the very nature of their job must spend hours in physical contact with known sick patients. The protocols must be strict because failure means guaranteed infection.

I on the other hand have no obligation to physically touch random strangers, thank god, and don’t have to go interact with known sick people. Instead I’m trying to provide some protection for myself, and trying to work in aggregate to reduce the risk that asymptomatic carriers spread the disease while doing essential activities. “Marginal benefits” in this context is also known as “reducing R0”, which is absolutely a public health goal.

Now all of that is a good argument for saving N95 masks for medical workers and relying on surgical or cloth masks for the general public, especially in the light of PPE shortages. But that is an entirely different argument from “masks don’t work”.

What's the point of wearing the mask then?

If you're wearing a mask to protect against droplets then maybe you'd get marginal benefits. But if your concern is virus carried in the water vapour that people exhale, then an N95 isn't nearly enough to protect you.

90% what?

It makes no sense to claim "90% effective".

90% reduction in numbers of people infected? Of droplets inhaled? Of droplets exhaled?

"Masks work, don't work" are not cogent arguments.

It's the same with plastic gloves. Their usage in ie. grocery stores etc. may not help as much as incorrect usage may work against the intention. Again the argument is that if you get your hands dirty, you know you need to wash your hands and might think twice before touching face. Even washing hands properly require some training.

We've since learned that less extreme measures may be good enough once you have control, testing and tracking.