A homemade cloth face mask likely needs two or three layers to be effective in preventing the spread of the coronavirus, according to an observational study published Thursday in the medical journal Thorax.

Researchers at the University of New South Wales in Australia looked at the effectiveness of single- and double-layer cloth face coverings at reducing the spread of respiratory droplets that pass when an infected person speaks, coughs or sneezes. They then compared those results with that of a three-ply surgical face mask.

The researchers used LED lighting and a high-speed camera to film the dispersal of airborne droplets produced by a healthy person with no respiratory infection. The video for speaking was captured at 850 frames per second, while coughing and sneezing was captured at 1,000 frames per second due to the higher expulsion speeds, they said. 

A three-ply surgical face mask was the most effective at reducing airborne droplet dispersal, the researchers found after reviewing the footage. The single-layer covering reduced the droplet spread from speaking, though the double-layer covering was better in reducing droplets from coughing and sneezing, according to the researchers.

“A cloth face covering with at least two layers is preferable to a single-layer one,” Prateek Bahl of the School of Mechanical and Manufacturing Engineering at UNSW, a lead author of the study, wrote in the findings. “Guidelines on home-made cloth masks should stipulate multiple layers.”

To be sure, the researchers said a single-layer face covering is better than no face covering at all. Additionally, several other factors determine the efficacy of cloth masks such as type of material, the number of layers, the arrangement of different layers and frequency of washing, they said.

The single-layer face covering was made from a folded piece of cotton T-shirt and hair ties. The double-layer covering was made using the sewing method as shown by the Centers for Disease Control and Prevention. The researchers used tissue paper to simulate the mucus membrane of the nasal cavity and induce a sneeze, according to the study.

The study comes as the Trump administration pushes for the public to adopt face coverings in an attempt to curb the spread of the coronavirus. The United States has the worst outbreak in the world with 4 million cases and at least 143,820 deaths, according to data compiled by Johns Hopkins University. 

President Donald Trump’s response to the pandemic has also come under increasing scrutiny. In recent weeks, Trump has downplayed the threat of the virus, tying the surge in new cases to an increase in testing. However, public health officials and infectious disease experts dispute those claims, saying the rate of cases that test positive in the U.S., hospitalizations and deaths remain high in some states.

Trump endorsed masks for the first time this week after resisting them for months. The CDC began recommending face masks for the general public in April. 

Earlier in the day, Adm. Brett Giroir, an assistant secretary for health at the Department of Health and Human Services, told reporters that 90% of the public or more need to wear face coverings in order to curb the spread of the virus. 

“If we close the indoor bars, reduce the occupancy of restaurants, have face coverings and hygiene, this is essentially equivalent to shutting down the entire economy, which means shutting down elective surgeries and everything else that goes with that,” he said.