A vast cloud of Sahara dust is blanketing the Caribbean as it heads to the US with a size and concentration that experts say hasn’t been seen in half a century. Air quality across most of the region reached record “hazardous” levels and experts who nicknamed the event the “Godzilla dust cloud” warned people to stay indoors and use air filters if they had them. “This is the most significant event in the past 50 years,” said Pablo Méndez Lázaro, an environmental health specialist at the University of Puerto Rico. “Conditions are dangerous in many Caribbean islands.” Many health specialists were concerned about those battling respiratory symptoms tied to the coronavirus pandemic. Lázaro, who is working with Nasa to develop an alert system for the arrival of Sahara dust, said the concentration was so high in recent days that it could even have adverse effects on healthy people. Extremely hazy conditions and limited visibility were reported from Antigua down to Trinidad & Tobago, with the event expected to last until late Tuesday. Some people posted pictures of themselves on social media wearing double masks to ward off the coronavirus and the dust, while others joked that the Caribbean looked like it had received a yellow filter movie treatment. José Alamo, a meteorologist with the US National Weather Service in San Juan, Puerto Rico, said the worst days for the US territory would be Monday and Tuesday as the plume heads toward the US south-east coast. The main international airport in San Juan was reporting only 8km (5 miles) of visibility. The mass of extremely dry and dusty air is known as the Saharan Air Layer and forms over the Sahara desert. It moves across the North Atlantic every three to five days during the northern hemisphere’s late spring to early autumn, peaking in late June to mid-August, according to the US National Oceanic and Atmospheric Administration. The layer can be 3km thick, the agency said.