Adequate ventilation in the home allows contaminants to leave the home rather than remain inside. Concerns about energy efficiency and rising energy costs have resulted in tighter homes with fewer leaks. Plus, people seldom their windows open these days. While homes may retain more constant indoor temperatures, they may also retain particles in the air that can irritate eyes and throats and exacerbate allergies and asthma. Professionally installed ventilation systems work to correct this situation by exchanging stale recirculated indoor air with fresh filtered outdoor air. They also help control excess humidity. Too much humidity can promote the growth of mold in both summer and winter. An efficient and well-maintained ventilation system works year-round to keep indoor humidity levels at a healthy and comfortable 30-60 percent.