Ozone shields our planet from the sun’s harmful ultraviolet rays, but it may be also be keeping bees away from their favorite flowers, according to a new study. Ozone doesn’t just live high in Earth’s atmosphere; near the ground, it contributes to smog, and ground-level ozone has gradually increased in most places because of industrial pollution from vehicles and fossil-fuel burning. Ozone is known to react with and break down molecules called volatile organic compounds. That’s a concern because flowers emit certain kinds of these compounds that, like a natural perfume, entice bees and other pollinators to visit the flowers. In the new study, researchers grew and collected black mustard (Brassica nigra, pictured), flowering plants that commonly appear around the world. In the laboratory, the scientists exposed groups of them to ozone levels ranging from 0 parts per billion (ppb) of ozone to 120 ppb, which is found in some urban areas. A machine called a mass spectrometer measured levels of scent molecules as far as 4.5 meters away from the flowers. At 4.5 m away, levels of most of the scent’s component molecules ranged from 17% to 31% lower at 120 ppb than at 0 ppb ozone, the researchers report online before print in New Phytologist. Little to no decrease occurred at 0 m away, showing that ozone’s effect increases with distance. Additional lab tests showed that unaltered scent is attractive to a common type of bumblebee (Bombus terrestris); bees preferred spending time in scent-laced air over scent-free air, the researchers say. Because some molecules’ levels decreased more than others in the lab tests, the scent’s strength isn’t all that’s at stake: The scent itself may change a bit, too. The findings suggest pollinators could struggle more over time to locate flowers, as ozone pollution in most parts of the world will keep rising as a result of industrialization and climate change.