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Homeowners often weatherize their homes, that is, they add insulation and seal up gaps to avoid drafts. In the past, many homeowners used insulation that releases formaldehyde, a gas that at high concentrations causes health problems. Such insulation is now banned. It is still possible, however, to create dangerous levels of formaldehyde by weatherizing a house, since reducing airflow increases the concentrations of any gas being released into the air of the house, and __________.