Exposure to smoke makes ragweed allergy worse.

There have been various reports on the effect of second-hand smoke on health. Now researchers at the University of California report on how it affects people with allergies. They recruited 19 nonsmokers with ragweed allergy and exposed them to either clean air or secondhand smoke for two hours.

A nasal lavage was carried out on each participant before and after the exposure. This was to assess the level of antibodies linked to immune response. After that, the participants were exposed to either ragweed or placebo and had lavage at ten minutes, 24 hours and four and seven days. Four days after exposure to either ragweed and secondhand smoke levels of antibody were around 16 times higher than they were after exposure to ragweed and clean air. This suggests that secondhand smoke can exacerbate allergic responses. Clearly, those with allergies need to avoid secondhand smoke.

Source
Journal of Allergy and Clinical Immunology 14th June 2006