Here’s why.
This outdoor wooden structure has no mechanical ventilation.
For the following simulation, we assume two air changes per hour based on gaps in the makeshift plastic windows and air leakage around the joints and door.
The wind speed is 0.5 meters per second which is considered to be light air on the Beaufort scale. With windows closed, the air would go around the structure and not through it.
With all windows in place, aerosols from an infected person who is eating and talking travel throughout the structure. For clarity, the simulation only shows 10% of the aerosols, which are not to scale, that the diner would exhale in 10 minutes.
For this simulation the front, left and right windows are removed.
The wind speed remains at 0.5 meters per second, but it now passes through the structure.
The volume of aerosols is cut by 4.4 times when the windows are removed. Once the infected person leaves, it would take less than two minutes for the infected aerosols to completely disperse.
“The reduction in particle concentration, in Simulation 2 would be similar to a conversation with an infected person where both are wearing surgical masks,” says Qingyan Chen, professor of Mechanical Engineering at Purdue University. “Simulation 1 is similar to a conversation with an infected person without wearing masks.”
In this outdoor tent-like structure, eight parties are seated more than 6 feet apart and the sides are removed.
Without walls, crosswinds will likely flush out potentially dangerous aerosols. “We are looking for enhanced removal and dilution of respiratory aerosols – that might contain viruses – by wind, which moves mainly in the horizontal direction,” says Linsey Marr.
As you add walls, the airflow is reduced. In this example the windows do not open. “If the wind is blowing left to right in this situation then it could create enough cross ventilation,” says Chen.
However, “if the wind is blowing front to back then it will go around and not through says,” Chen.
Enclosing the majority of the structure will block most crosswinds but “air leakage will be better than being indoors without mechanical ventilation but much poorer than indoors with mechanical ventilation,” says Chen.
Igloo style tents can be a safe alternative if used correctly. The safest approach is to limit the party to members of your household, since you are already sharing germs with them.
If you are with an infected person who is not in your household, then the risk is significant. Since the structure is sealed tightly, virus particles can build up over time.
After a party has left, there needs to be time for the structure air out. In addition to wiping down surfaces, Marr recommends opening the vents and door for 15-30 minutes to allow aerosols to deplete.
Single-party, greenhouse dining pods have become a popular choice for restaurants because of their size and affordability – some can be found for under $800.
Several modified versions feature half-doors that allow the wait staff to converse with guests and deliver food without coming inside the structure. But like the igloo, there may be some risks.
If you're dining with members of your own household, however, these structures can also protect you from other patrons.
As with the igloo-style pod, make sure it’s cleaned and aired out for 15-30 minutes between parties, says Marr. Several models have louvered roof panels that open, making them easier to air out.