Today, in the polluted air found downwind of large metropolises, scents may only make it some 650 to 980 feet (200 to 300 meters).
The impact is especially pronounced during high-pollution "code red" days in summer.
"Lots of vehicles are releasing nitrogen oxides," Fuentes said. "When [the gases] are in the presence of sunlight they are converted into these molecules that we call ozone—one of the main pollutants that we find in the eastern U.S. in the summer months.
"Fragrances are overwhelmed by it."
Fuentes and colleagues published their findings recently in the journal Atmospheric Environment.
(Related news: "Japan's Cherry Trees Bloom in Fall; Warming to Blame?" [November 27, 2007].)
Bad News For Bees?
With bee populations dropping dramatically in many parts of the world, could these missing scents be a factor?
Scientists trying to pinpoint the cause of bee declines have variously blamed viruses, mites, bacteria, pesticides, and even cell-phone radiation.
Jay Evans, an entomologist at the U.S. Department of Agriculture's bee research laboratory in Beltsville, Maryland, was intrigued by the new study but hasn't seen bee behavior that suggests trouble with scents.
"Over the last couple of summers I don't think the bees in this area were bringing in much less food," he said.
"It might be that they had to work harder, but it seems like as long as there were bees to collect food they were finding flowers somewhere."
Evans also noted that beekeepers didn't report big drops in their honey yields, which would have occurred had food been harder to find.
But lead author Fuentes fears that the fading smell of flowers may stress insects already faced with an array of other threats.
"The [effects shown in] these studies will simply exacerbate whatever the bees are going through right now," he said.
"It's something that is really worthwhile paying attention to."
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