talking about the importance of wearing a mask. i know you spoke to one of the scientists raising the alarm with the world health organization. that scientist telling you, wear masks indoors, even in public places where they've done social distancing, like restaurants that have put tables far apart. they said wear masks anyway. >> reporter: yes, david. and this is why. one study suggesting that these particles can linger in the air for as long as 16 hours in a lab setting. dr. fauci and dr. marr say this reiterates why it's so important to wear a mask, especially indoors, even when social distancing. david? >> and it just reminds us that there is still so much we don't know months into this. eva, thank you. and tonight, the news from brazil, where the president of that country has repeatedly downplayed the severity of this pandemic. well, today, he revealed he has tested positive. wearing a mask before the cameras, but then taking it off. here's our foreign correspondent james longman tonight. >> reporter: he played down the dangers of covid, but tonight, brazil's president jair