IntroductionWe spend most of our time indoors, at work, at home and at school or even at mall. There are many sources of indoor air pollution; building materials, cigarettes, consumer products, gas appliances and furniture can all make the problem worse. However, the toxic emissions of many of these contributors are unmeasured or only partially measured by local, state, or federal laws. Pollution from power plants, cars, and other transportation is a well-known source of outdoor air pollution, but sometimes our indoor air quality is worse; it can be up to ten times worse for you than the air outside. The U.S. Environmental Protection Agency's revised National Ambient Air Quality Standards (US EPA NAAQS) for PM2.5 and other air pollutants may be exceeded not only outdoors, but also inside. This is based on the various studies on indoor and outdoor particles in the city center environment or even near busy main roads. In recent years, indoor air quality (IAQ) has received excessive attention. The main reason is that we spend a lot of time indoors in modern times. City dwellers spend on average 87% indoors and only 6% outdoors. We also believe that the indoor environment is better and more pleasant to live in than the outdoors. We also believe that indoor air is cleaner, more relaxed and healthier and that the building will protect us from harmful substances present in the surrounding environment. For this reason, a number of air quality warning systems around the world, also designed for outdoor use, provide warnings or advice during episodes of poor air quality. air, to stay inside. Yet the important question is: is indoor air really cleaner? Is it free of external pollutants?...... middle of paper ......al. “Relationship between indoor and outdoor exposure to fine particles near a busy thoroughfare.” Indoor Air 9.2 (1999): 75-84.3. www.epa.gov/iaq/ia-intro.html4. http://www.arb.ca.gov/research/indoor/indoor.htm5. Li, Wen-Whai et al. “Correlations between short-term indoor and outdoor PM concentrations in residences equipped with evaporative coolers.” Atmospheric Environment 37.19 (2003): 2691-2703.6. Leader, Brian P., et al. “Indoor, outdoor, and regional summer and winter concentrations of PM10, PM2.5, SO4(2)-, H+, NH4+, NO3-, NH3, and nitrous acid in homes with and without kerosene heaters.” Environmental Health Perspectives 107.3 (1999): 223.7. www.umad.de/infos/aqm2005/IndoorAirQuality.pdf8. http://www.treehugger.com/natural-sciences/indoor-air-quality-causes-of-testing-and-monitoring-indoor-air-pollution.html9. http://www.epa.gov/heasd/research/emi.html