Achoo!

Over the past few decades, allergy season has gotten more severe—and artificial lights may be partly to blame.

Plants rely on signals like days growing shorter in colder months and longer in warmer ones to know when seasons are changing. But artificial lights at night can trick trees into thinking days are longer than they actually are. Recently, environmental scientists at Vanderbilt University in Tennessee found that trees in bright cities are producing leaves and flowers up to nine days earlier in the spring and losing their leaves almost a week later in the fall compared with trees that aren’t exposed to artificial light.

The amount of artificial light at night is increasing each year. That may continue to lengthen plants’ growing season and cause them to release their pollen earlier, triggering more intense seasonal allergies.