Dust clouds from the Sahara have again made their way to Cuba, causing hazy days for the island.

Scientists warned of the occurrence at the start of the month, according to Xinhuanet.

Each year, particles from African sandstorms are transported across the Atlantic Ocean and into the Caribbean, which is problematic for pilots and asthmatics, according to The Associated Press.

The dust clouds settle over Cuba and cause intense orange-tinted sunsets and hazy days. The yearly event can be traced back to the time of Charles Darwin in 1832. The dust has been analyzed, and scientists have found metal, microorganisms, bacteria, spores, pesticides and fecal matter in the dust, but not enough for it to be a health risk.

The phenomenon has been appearing in the U.K. as well, according to the Huffington Post.

Because it is so unusual, children were kept home from school in the U.K. this year amid confusion over the extent of health risks from the clouds, according to the Telegraph.

One government department warned of very high levels of pollution from the dust clouds, and another said there would be very little risk and there was no need to stay indoors. But schoolchildren were kept indoors anyway.

Researchers have for years been analyzing the dust in the Caribbean, in places like Barbados and Cuba. Cuba's research has helped determine what types of over-the-counter medications are needed for those who suffer from respiratory diseases, including asthma.

Additionally, the clouds trap heat and prevent rain from falling and hurricanes from forming.

In 2013, the dust clouds traveled to Wyoming, which was unexpected.

Scientists say that in that last three decades, the amount of dust arriving in Cuba has increased tenfold because of severe droughts in northern Africa.