The Centers for Disease Control and Prevention is now recommending face coverings for vaccinated people in certain situations. Please tell me exactly when to mask up.

If you hung your mask up in May after the Centers for Disease Control and Prevention said face coverings were no longer necessary for vaccinated people, you're probably not eager to start masking up once more. And with Tuesday's announcement that some vaccinated people should mask up again in certain situations because of the spread of the highly contagious delta variant, your head may be spinning.

The new guidance is targeted at vaccinated people who live in areas with "high and substantial transmission," and it focuses on indoor settings.

Given that the recent rise in hospitalizations is probably because of people no longer wearing masks at the appropriate time, says Charlotte Baker, assistant professor of epidemiology at Virginia Tech, many health experts welcomed the reversal.

But there are many unanswered questions as people look for guidance in specific situations: Do I really need to mask up at the grocery store? How about on a college campus that requires students to be vaccinated? What about roller coasters?

"It's so subjective and situational," says May Chu, a clinical professor of epidemiology at the Colorado School of Public Health, who led the research on masks and respirators for the World Health Organization. "It's easier to think it through if you know what the risks are that you need to evaluate."

To help you assess and mitigate your risk in specific situations, we asked specialists in ventilation, masking, public health and infectious disease for their input. They recommend thinking about the following questions when deciding whether to doff or don. Warning: The first few questions have easy answers, but they get trickier.

Are you vaccinated?
This is one of the only clear-cut answers: If you haven't gotten jabbed yet, your risk in any public situation is high enough to wear a mask always.

Will everyone else there be vaccinated?
On the flip side, when you're sure everyone else is vaccinated, experts agree the risk is so low there's no need to mask.

Do you have a cold, the flu or COVID-19-like symptoms?
If you do, quarantine at home (and get tested if you have symptoms of COVID-19). And if you have just a garden-variety cold or flu and have to go out, wear a mask for the health of everyone around you.

Are you going to be outside?
Virus particles dilute rapidly outdoors. Virginia Tech aerosol expert Linsey Marr has compared it to a droplet of dye in the ocean: "If you happen to be right next to it, then maybe you'll get a whiff of it. But it's going to become diluted rapidly into the huge atmosphere."

Our experts believe that most vaccinated people are safe without masks in most outdoor situations without prolonged close contact.

These next questions don't come with neat, stand-alone answers. Consider each response as one piece of the puzzle as you make your mask decision.

Are transmission rates low to moderate in your county?
In areas with high rates of vaccination, test positivity rates have fallen to the lowest levels since the beginning of the coronavirus pandemic ( 5% is considered low). With low levels of virus circulating, going out without a mask is much safer. However, keep your eye on that rate as cases start rising again because of that delta variant, says Stanford University infectious disease fellow Abraar Karan. "The change over time will tell you in which direction the epidemic is heading," he says.

Whom do you live with?
If you live with unvaccinated people (including children) or with people 65 and older or anyone else especially susceptible to the coronavirus, wearing a mask could reduce your risk of developing a rare breakthrough infection and passing it on to them.

"If you live in a high-risk area and with vulnerable elderly folks who haven't been vaccinated or even if they have, I would recommend masking up indoors in close spaces until we have more data about the necessity for boosters [for older adults and other vulnerable populations]," Karan says.

Will it be so crowded that you cannot maintain 6 feet of distance between people?
If you're shoulder to shoulder with people, it's prudent to wear a mask depending on how long you're there and especially if you're in a higher-prevalence area, Chu says.

Will people be singing? Cheering? Screaming? Exercising?
Even when you're inside, there are different considerations. Consider a sports bar with people shouting and cheering, a bar with a crowded dance floor and people singing versus a silent prayer meeting, Chu says.

Are there open windows if you're inside?
That's an indication of good ventilation — most people won't know whether the air change rate is more than four times per hour or whether the venue has MERV + filters or portable HEPA air filtration units, Marr out.

High ceilings are also a bonus: That's an indication that there's good potential for dilution, Marr says.

The bottom line
None of the experts has tossed their masks. Baker, the Virginia Tech epidemiology professor, continues to mask up whenever she is indoors with anyone who may not be vaccinated. "I feel really naked not wearing one," she says.

"Erring on the side of caution doesn't hurt anybody," says Jessica Malaty Rivera, an infectious disease epidemiologist who has worked with the COVID Tracking Project. "And wearing masks does not deny the effectiveness of these vaccines."

And as for roller coasters? It's a tricky one, Baker acknowledges, since you're outside but close to others. If you knew everyone on the ride was vaccinated, it'd be OK to skip the mask. In lieu of that?

"I would wear a mask," she says.

Sheila Mulrooney Eldred is a freelance health journalist in Minneapolis. She has written about COVID-19 for many publications, including Medscape, Kaiser Health News, Science News for Students and The Washington Post. More at sheilaeldred.pressfolios.. On Twitter: @milepostmedia.