Outdoor dining is allowed everywhere in California, but is it safe?

SAN FRANCISCO (KGO) -- With the vast majority of California's population back in the purple tier, outdoor dining is the name of the game for restaurants. But as the weather turns, those businesses are trying to adapt once again to the rules around safe dining in the COVID-19 era. Diners, too, are wondering just how safe it is to dine outdoors while cases are spiking nearly everywhere.

Here are the rules around outdoor dining in California and the Bay Area, and what you need to know when considering it.

Where is outdoor restaurant dining allowed? Is indoor restaurant dining allowed anywhere?

Outdoor restaurant dining is allowed in every reopening tier in California, including the strictest "purple tier" status. Indoor dining with limited capacity is allowed starting in the red tier according to the state's timeline, but counties can always take it slower.

As of Thursday in the Bay Area, outdoor dining is allowed in all nine counties. Indoor dining is only allowed in San Mateo County at 25% capacity.

INTERACTIVE: Here's the reopening status of every Bay Area county

You'll remember, in the beginning of the pandemic restaurants were only allowed to do takeout -- no indoor or outdoor dining. In theory, any county could revert to those stricter stay-at-home order restrictions, but no Bay Area county seems to be considering it at this time.

What health and safety guidelines are restaurants required to follow for outdoor dining?

The state has a 13-page document of guidelines restaurants have to follow to reopen, whether it's for indoor or outdoor service. They include: Staff have to wear masks at all times, diners have to wear masks when not eating, tables must be spaced out and cleaned after use.

Are tents and igloos allowed? Is eating under a tent or in an igloo safer than indoor dining?

Outdoor tents, igloos and geodesic domes are all allowed in California -- and you'll probably see more and more of them pop up as the weather gets colder. But when it comes to safety, the key thing here is ventilation and airflow. Local health departments can also fine restaurants if their outdoor setups aren't up to code.

A tent that's open on two sides and allows a breeze to blow through is safer than a full enclosed dome, for example, because it's less likely that viral particles in the air will linger.

RELATED: Rohnert Park restaurant owner fined $1,000 for unsafe outdoor tent amid COVID-19

"So if someone is infected and they're sitting outdoors in one of these tents, when they're breathing and talking, they release viruses into the air and, just like cigarette smoke, the air can easily flow in any direction because there are no walls," Virginia Tech Professor Linsey Marr told ABC News. "And once you start adding walls, you potentially block that wind. Once you add four walls, you kind of lose that benefit of being outdoors."

Fully enclosed domes and tents protect you from others, so in that sense they might be safer if you're only dining with members of your household. However, if someone in the enclosed space happens to be infected, they are much more likely to transmit it to others.

A sushi restaurant in San Francisco that installed outdoor plastic domes on its patio was ordered to take them down because they lacked ventilation. They have since modified the structures and reinstalled them.

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