Wearing a double mask provides twice as much protection against COVID-19 as a single one, according to a new study by the US Centers for Disease Control and Prevention (CDC).

In the study released Wednesday, two artificial heads with a breathing function were outfitted with one masks or two, and particles the same size as COVID-19 particles were sprayed on them from a distance of about 2 meters (6.5 feet).

The researchers found that 40% of the particles sprayed on the artificial head wearing one mask were blocked, while 80% were blocked on the artificial head wearing two.

If everyone in the same environment wears a double mask, the amount of particles blocked increases to 95% in total, said Dr. John Brooks from the research team.

“The first challenge is getting as many people as possible to wear masks. After that, to help those who wear a mask get the most out of that mask,” said Brooks.

The CDC has updated its anti-virus guidance that a cloth mask worn over a surgical mask can be more protective by closing gaps in the initial mask.

But some factors in the study tended to muddy the conclusion, said a CDC press release. As the study was conducted with one medical mask and one cloth – among many different ones available – it said: “The findings of these simulations should neither be generalized to the effectiveness of all medical procedure masks or cloths masks nor interpreted as being representative of the effectiveness of these masks when worn in real-world settings.”

The findings also might not be generalizable to children or to men with beards and other facial hair, said the CDC.

It added that controlling the virus’ spread “is critical not only to reduce the widespread effects of the COVID-19 pandemic on human health and the economy but also to slow viral evolution and the emergence of variants that could alter transmission dynamics or affect the usefulness of diagnostics, therapeutics, and vaccines.

“Until vaccine-induced population immunity is achieved, universal masking is a highly effective means to slow the spread of SARS-CoV-2 when combined with other protective measures, such as physical distancing, avoiding crowds and poorly ventilated indoor spaces, and good hand hygiene.”