limited difference and little sense in terms of the money you invest. Once you decide you want to improve the quality of air in your whole house and not just one room, the next step is to decide which danger levels of pollutants you want to control Danger Level 1, Danger Level 2, and/or Danger Level 3. If your home was built in the last 30 to 40 years, it's referred to as a "tight home" because it was built to conserve energy by keeping cold air out in the winter and hot air out in the summer. But that weather-tight efficiency brings about a whole new problem - indoor air pollution. How does it happen if your home is supposedly weather-tight? A home normally breathes, allowing air to flow in and out. By design, a tight home reduces this air exchange. As a result, indoor allergens and particulates become trapped inside and start accumulating in high levels potentially causing health problems.