At a time when a mask has become a political statement — at least in the US — it isn’t surprising that the semantics between a droplet (a small drop of liquid, just to clarify)and an aerosol (a suspension of either solid particulate matter of liquid droplets in gas or, in this case, just air) should become a matter of great debate.

Which could explain the enthusiasm with which the US Centers for Disease Control and Prevention’s Friday update to its website on “how Covid-19 most commonly spreads” was received. And which could explain why, less than 15 hours after it was widely reported (early on Monday morning India time), the update was pulled, with CDC claiming it was a draft that was posted in error.

“Through respiratory droplets or small particles, such as those in aerosols, produced when an infected person coughs, sneezes, sings, talks, or breathes. These particles can be inhaled into the nose, mouth, airways and lungs and cause infection. This is thought to be the main way the virus spreads,” the now-rescinded update said. “There is growing evidence that droplets and airborne particles can remain suspended in the air and be breathed in by others, and travel distances beyond six feet (for example, during choir practice, in restaurants, or in fitness classes). In general, indoor environments without good ventilation increase this risk,” it added. The language was very different from that of CDC’s previous update (in June) on how Covid-19 spreads; that June update, now back on the organisation’s website, said nothing about aerosols.

Because some may miss it, let me point to the “distances beyond six feet” in the rescinded update. I’ve seen people in conference rooms remove masks citing the six feet rule, and continue to talk (loudly). Both, the update-that-wasn’t suggested, are risky. So is the 15-minute exposure threshold CDC suggested in a late July note. I know of organisations that keep meetings to under 15 minutes purely on the basis of that — the internet is a wonderful thing, but as the poet Alexander Pope put it, “a little learning is thing; drink deep, or taste not the Pierian spring.”

The CDC’s new guidelines-that-weren’t clearly pointed to the heightened risk of indoor transmission, especially in shared rooms, but also in open offices with poor air conditioning and bad ventilation. And talking (especially loudly, which is a natural reaction in conference rooms because everyone is spread out), was an additional risk. There’s thus far been an assumption that it is all right to speak without masks in socially distanced meetings (usually held in a closed room). Not so, indicated the CDC update. Going by it, even plexiglass-separated lawmakers in the current session of Indian Parliament are not safe. Most of them who speak do so without their masks, making it possible for aerosols to circulate (the plexiglass-glass is in front and on the side, not on top).

The World Health Organization has been importuned for months by scientists who have asked to change its own advice on transmission to include the warning on airborne transmission. That hasn’t happened. While it isn’t clear what made CDC change its mind, its rescinded update can still form the basis of guidelines that governments around the world can now draft — to ensure the safe reopening of public spaces and the restart of activities. There’s enough evidence (based on research) to prove the airborne transmission of the virus.

These guidelines will need to cover two aspects — the physical, and the behavioural. The first covers such things as nature of ventilation and air-conditioning, the number of people present and the distance between them, and presence or absence of barriers(such as plexiglass partitions). The second has to do with whether or not people are wearing masks, the amount of talking (or shouting or singing), the duration of the gathering or meeting, and the nature of the activity in which people are engaged (for instance, a gymnasium, where a bunch of people are exercising, and, consequently, breathing heavily, is definitely more risky than a quiet ride in the Metro).

This may sound like a lot of detailing, but with a vaccine unlikely to be available till the middle of 2021, and life (and work) having to go on, everyone has to learn to live with the virus — and that involves rules, a lot of them, but all based on data and science, even if politics sometimes gets in the way.