Here are my heuristics about behaviors that are safer/riskier while the coronavirus is still a threat. This is NOT advice you should take; it is a description of what I personally am comfortable with at this point in time. (Standard I AM NOT A DOCTOR/EPIDEMIOLOGIST/DISEASE EXPERT IN ANY WAY warning) I’m focused on behaviors that I expect to make decisions about on a day-to-day basis. It is very important to note that all of the below advice is for a LOW-RISK person; anyone who is high risk should probably categorize this very differently.

Assumptions:

• There is a dosage effect: being exposed to lots of virus particles at once is a lot more dangerous than being exposed to few particles over a long period of time. My mental model of it is that your body can fight off a small number of particles easily because they can contain the threat before the virus has a chance to start replicating significantly.
• The virus is expelled by infected people in both large and small droplets (e.g. I believe there is more of an aerosol effect than originally believed), which can float through the air bound on air currents for an unknown amount of time before “dying”.
• The virus is more quickly destroyed in sunlight than outside.

Relatively safe for low-risk people right now:

• Walking around outside alone on the sidewalk, even without a mask - it seems very unlikely to me that you’d get infected walking by someone else. (In practice, I always bring a mask with me and mask up when approaching someone I’m about to pass by, but that’s because it’s now required by my county and I’m afraid of being called out, not because I think it’s effective.)
• Going to parks and generally spending time outside, even without a mask. (same caveat as above)
• Exercising outside, even without a mask.
• Picking up food and items curbside with a mask, assuming people aren’t hanging around densely outside the pickup spot.
• Eating takeout food: I haven’t heard of any cases transmitted through food prep, though intuitively it seems like it should be possible. This is also dependent on whether the restaurant incentivizes sick workers to come in anyway; having information about restaurant employee practices would make me feel more confident about this one than I am right now.
• Walking around outside with a mask with people not in your household: This is probably the most controversial thing on the list, but it seems to me that walking around outside, preferably a few feet apart, with a mask on, is a pretty safe way to see friends and ease the burden of social distancing.

Borderline for low-risk people right now:

• Doing errands anywhere indoors, even with a mask. This seems heavily dependent on how many virus particles a sick person expels in, say, 15 minutes inside a grocery store, how long those particles stay active, and how likely airflow is to push those particles to someone else.
• Picking up food and items curbside with a mask, with people hanging out densely outside the pickup spot.
• Traveling on BART: ridership has dropped so much that I doubt you’d be exposed to many people (last time I took it sometime in April, there were only two other people in the car during what would normally have been rush hour). However, would those people be more likely to be sick? I also don’t know how to model BART cars as “inside” vs “outside” - given how often the doors open and close, I could guess that there’s enough airflow that it basically counts as “outside”, but the lack of sunlight and long stretches of enclosement (across the bridge) would make it worse. Overall, I think you’d be unlikely to be infected from rare, short BART trips, but I have pretty low confidence about this one.
• Taking Lyfts/Ubers. I have taken Lyfts home after doing groceries because no one in my apartment has a car and it feels less risky than BART, but I think

Things that are relatively risky for low-risk people right now:

• Going to restaurants or anywhere indoors for long periods of time. That means no eating out, trying to get through grocery shopping quickly, no visiting friends’ apartments, no plane rides, etc.

I expect it’ll be interesting to revisit this over time and see what changes; this list is already pretty different from the one I would have written two months ago.