Restaurants are setting up outdoor tents to accommodate diners as the weather gets colder. But these heated structures may not offer the same protection from COVID-19 as open-air dining areas, experts say.

They can present a false sense of security to those who follow the “outside is better than in” rule because the enclosed tents can pose as many risks as indoor eating.

According to The Wall Street Journal, while transmission risk of the virus is higher indoors than outside, tents offer poor ventilation and unmasked diners in them are often packed close together.

“As structures become more fully enclosed, they become the same as an indoor environment or in some cases worse, with a relatively closed volume of air,” said Jeffrey Siegel, Ph.D., a professor of civil engineering at the University of Toronto.

He told the Journal that the lowest risk outdoor dining would feature tables spread apart and heating elements to offset the cold. He recommended that outdoor setups such as tents have at least two exposed sides that offer a cross breeze and diners sit closest to any opening that offers fresh air. Siegel warned that outdoor, enclosed structures may actually be worse than dining inside a restaurant, where powerful HVAC units, air filters, and better air flow decrease the risk of transmission.

Some restaurants have opted to purchase igloos for their outdoor dining patrons this winter. Geodesic pop-up domes, also called dining igloos, are made of clear plastic and snap together, and offer private dining areas for families and friends. They can be heated with space heaters.

According to the Journal, Iahn Gonsenhauser, M.D., chief quality and safety officer at The Ohio State University Wexner Medical Center, said that dining in individual igloos, domes, or even a greenhouse may offer a reduced risk of transmission, as long as people remain at least three feet from others and wear a mask when they’re not eating or drinking.

He also recommended airing the structure for at least 15 minutes between parties and wiping down all surfaces including the walls. Dr. Erin Bromage, a comparative immunologist and professor of biology at the University of Massachusetts Dartmouth, said that while you can get the virus if an infected person coughs or sneezes nearby, you can also get sick if you are in an enclosed space for a long period of time where the virus has been released by someone just breathing into the air. In an article for Occupational Health & Safety, Bromage said a single sneeze can release 30,000 droplets.

“Enclosed spaces defeat the purpose of outdoor seating for COVID-19 purposes,” said Gonsenhauser, according to the Journal.