Indoor Socializing
Indoor Socializing
[White Hat and Cueball are having a conversation.] White Hat: How are you? Cueball: Excruciatingly aware of how much of each other's gross lung air we're breathing. Cueball: I mean, fine!  How are you?

Cueball is meeting White Hat , who is probably not in the same household. White Hat asks how Cueball is, which is normal small talk, but Cueball responds by expressing his anxiety that they're inhaling one another's "gross lung air". Cueball then repeats a common joke regarding how impossible it is to connect with people when our conversation norms discourage honest communication, switching to a more socially acceptable "fine".
"Gross" here may be a pun on the term gross anatomy (i.e. anatomy at the macroscopic level) and "gross" as a synonym for "disgusting."
A recurring theme in xkcd is characters expressing an uncomfortable awareness of realities that most people tend to ignore, particularly for experts in a particular field (examples include 2057: Internal Monologues , 913: Core , 203: Hallucinations , and 1839: Doctor Visit ).  In this strip, likely as a result of being primed by awareness of the COVID-19 pandemic , Cueball finds it difficult to be in the same building with other people without being aware of the fact that they're breathing the same air, meaning that particles of biological material are being freely exchanged. In an earlier era, such concerns might have been dismissed as being extreme, but the pandemic has demonstrated that there's very real reason to be concerned.  Even if everyone involved is vaccinated, that doesn't entirely remove the risk, nor does it protect against other diseases, which can spread in similar ways.
The title text reinforces the idea that knowing more about any subject increases the likelihood that you'll become disturbed by some constant and basic reality of life. In this case, studying biology tends to be disturbing, since the field involves in depth knowledge of our own bodies, as well as all other organisms we encounter, and which makes one uncomfortably aware of all the risks and flaws basic to being alive.
Normally, inhaling unfamiliar biological organisms from the bodies of others is one way the immune system learns its environment, to prepare for possible diseases like seasonal colds.  With the advent of common distant travel, culture has adapted to the onslaught of new organisms people are exposed to, giving us strong senses of hygiene to protect our health beyond the adaption of our immune systems.  Diverse cultures of hygiene have evolved deadly superbacteria, produced sets of people with very good hygiene and very weak immune systems, as well as saving millions of lives, providing for treatments like safe open surgery and normalising novel piercings.  Often learning of the realities of the pervasiveness of micro-organisms and the details of biology can clash with one's culture of hygiene.
[White Hat and Cueball are having a conversation.] White Hat: How are you? Cueball: Excruciatingly aware of how much of each other's gross lung air we're breathing. Cueball: I mean, fine!  How are you?
