Greasy hair might not give you an attractive appearance, but it might reduce the amount of ozone you breathe in, and thus prevent you from respiratory problems, according to scientists.
The findings are based on the study which was conducted by Lakshmi Pandrangi and Glenn Morrison from the University of Missouri in Rolla.
In the study, the researchers exposed eight washed and eight unwashed hair samples to ozone for 24 hours.
The analysis revealed that, on average, unwashed hair absorbs around seven times as much ozone as freshly washed hair.
"Ozone is probably reacting with components of hair oil," New Scientist quoted Morrison, as saying.
Ground-level ozone can cause respiratory problems and has been associated with increased mortality.
Morrison said that having greasy hair could reduce your ozone exposure if you are indoors.
"For dirty hair, the ozone concentration around the head is likely to be substantially lower than the level in the room," he said.
However, just before you throw out your shampoo, the researchers found that unwashed hair samples produced more secondary-reaction products, such as the respiratory irritant 4-oxopentanal, because of the ozone reacting with the hair oil.
Since elderly and sick people are particularly vulnerable to the effects of air pollution, Pandrangi and Morrison suggest that indoor air should be filtered to reduce ozone, rather than focusing all our efforts on cleaning up ozone smog.