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.

“The performance of double masking simply does not add up,” wrote the researchers, led by Makoto Tsubokura.