A recent study appears to shed light on the issue: The problem is likely due to mold allergy. Researchers in Connecticut found that after 2 weeks of being indoors, a live Christmas tree emitted significant amounts of mold spores into the air. In fact, the amount of mold spores found in a home with a Christmas tree was nearly 10 times the amount of mold normally found inside. People who suffer from mold allergies may want to limit the amount of time a Christmas tree is kept indoors (such as less than 2 weeks), or consider using an artificial tree.
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