A major dust storm is in progress for Puerto Rico and the surrounding islands in the Greater and Lesser Antilles. Concentrations of up to 600 ug/m3 at the surface are forecast by the US Navy dust model. While it’s unlikely concentrations this high will be observed, note that the US EPA Air Quality Index lists concentrations over 425 ug/m3 as hazardous to human health.
Visibility has decreased to six miles at the San Juan airport as of 10pm Atlantic Standard Time. The National Weather Service forecast discussion has information on what their meteorologists are thinking about this event:
Re: National Weather Service Map Discussion – look esp. in aviation section: https://forecast.weather.gov/product.php?site=sju&product=AFD&issuedby=sju
Saharan Dust can reduce visibility (as is it today in San Juan, Puerto Rico), and at high concentrations cause health effects and hinder coral growth. The stable, dusty air also suppresses hurricane formation.