Dear Tom,
I have been keeping records of the pollen count for many years, and the pollen level has
been increasing. What weather factors are causing this?
--William Sanders, Chicago
Dear William,
It's not the weather but rather higher levels of carbon dioxide in the atmosphere that
have markedly increased plant growth rates, including ragweed (the source of pollen
that triggers hay fever). Since 1900, atmospheric carbon dioxide has increased 30
percent, from 294 parts per million (ppm) to 380 ppm now. In response, ragweed has
doubled its pollen production since 1900. Dr. Paul Epstein of Harvard University's Center
for Health and the Global Environment says, "This carbon dioxide stimulation of plants
has a side effect for public health. The hyper-production of pollen can cause hay fever,
conjunctivitis and can exacerbate asthma."
