Japanese supercomputer simulations showed that wearing two masks gave limited benefit in blocking viral spread compared with one properly fitted mask.

The findings in part contradict recent recommendations from the U.S. Centers of Disease Control and Prevention (CDC) that two masks were better than one at reducing a person's exposure to the coronavirus.

Researchers used the Fugaku supercomputer to model the flow of virus particles from people wearing different types and combinations of masks, according to a study released on Thursday by research giant Riken and Kobe University.

Using a single surgical-type mask, made of non-woven material, had 85% effectiveness in blocking particles when worn tightly around the nose and face. Adding a polyurethane mask on top boosted the effectiveness to just 89%.

Wearing two non-woven masks isn't useful because air resistance builds up and causes leakage around the edges.