The efficacy of face masks for limiting the spread of SARS-CoV-2 remains uncertain and hotly contested. Recommendations vary between countries as do the reasons given. In Norway, where I live, masks are not considered necessary because very few people are infected, and efforts to contain the spread of the virus have been quite successful without mandating their use by the general public. But the debate about whether or not masks “work” is complicated and requires attention to numerous variables and contingencies. Even if we can agree that masks do help limit the spread of the virus to some degree, the conditions under which they are most effective can differ enormously, and recommendations to wear masks “in public” can actually mislead more than they illuminate and even cause some harm. We now know more than we did even a few weeks ago about how the virus spreads. In a blog post inspired by a Quillette essay analysing superspreader events, immunologist Erin Bromage delineates the salient principle as “Successful Infection = Exposure to Virus x Time.” Exposure …