DETROIT — Studies conducted by Wayne State University physician researchers indicate that combining face shields and masks make little difference.

The university’s School of Medicine performed studies that indicated that the use of plastic face shields with surgical masks provides the best protection against COVID-19, but combining the two made little difference over using just masks alone.

“Surprisingly, when assessing the combined protection of surgical mask and face shield, the level of protection was not enhanced in comparison to using surgical masks alone,” said Dr. Teena Chopra, Wayne State University professor of internal medicine and infectious diseases. “This result reinforces the fact that a surgical mask serves as an effective intervention, providing protection from aerosol droplets that spread COVID-19.”

In their tests, Chopra and Dr. Hossein Salimnia, a professor of pathology, created an aerosol-generating device consisting of a nebulizer, oxygen tank and fan to stimulate an infected human speaking to an uninfected human from a distance of 0.15 meters and 1.82 meters.

The doctors then measured the number of bacterial colonies on agar plates to test the effectiveness of masks, face shields and the two combined.

When the shield was used alone, the researchers found significant numbers of bacterial colonies developed on the agar plates.

When only a mask was used, the plates developed minimal bacterial colonies.

The researchers found no significant improvement when face shields were used in combination with surgical masks.

Evidence suggests that SARS CoV-2, the virus that causes the COVID-19 infection, is transmitted when infected people expel virus-containing droplets by sneezing or coughing.

Studies also show that breathing and talking can also expel droplets that contain the virus.

“This simulation indicates that surgical masks worn by all are more likely to provide significant protection to uninfected participants in indoor conversations,” Dr. Chopra said. “The role of the face shields in preventing transmission is not supported; and if transmission via the eyes is felt to be a significant factor, such as with health care workers and dentists and dental technicians, goggles might provide better protection than a face shield.”