The debate over “airborne” coronavirus spread, explained

The WHO says aerosol Covid-19 transmission “cannot be ruled out.” Here’s what that means.

Since the beginning of the pandemic, the World Health Organization has stressed that, primarily, Covid-19 spreads through very close personal contact. The virus-laden droplets exhaled from a sick person’s mouth and nose, the thinking goes, are heavy, and fall to the ground before they can get much farther than 6 feet.

But as the pandemic has unfolded, contact tracing studies have shown this isn’t always the case. In China, an air conditioner pushed virus-laden air across...