"This section of the standard defines its scope in a number of different ways. First@ the scope describes where the standard is intended to be applied in terms of the types of buildings where it should be used. Second@ the scope describes what the standard addresses@ that is@ what kinds of issues in the house affect IAQ. This includes a list of reasons why acceptable IAQ may not be achieved@ even if all of the requirements of the standard are met. Third@ the scope lists equipment that is not addressed by the standard. The standard applies to all residential spaces intended for human occupancy in single-family houses or in multifamily buildings up to three stories. The phrase intended for human occupancy means any space that people will normally enter. This includes: PURPOSE This section of the standard does not contain any requirements. It does describe why the standard exists and what compliance with the standard is expected to achieve. The purpose is a single sentence@ but it has two parts. The first part@ ""defines the roles of and minimum requirements for mechanical and natural ventilation systems and the building envelope@"" describes the parts of the building that the standard will primarily be addressing with its requirements: the ventilation system (mechanical or natural) and the building envelope. Do not interpret this as a statement of the scope of the standard; it includes requirements for additional parts of the building too (see Section 2@ ""Scope@"" for more on this). The second part of the purpose@ ""intended to provide acceptable indoor air quality in low-rise residential buildings@"" describes why the requirements of the standard exist. Acceptable indoor air quality is a term that is defined in Section 3@ ""Definitions@"" to mean air that is neither irritating nor unhealthy. Indoor air that is not acceptable is air that smells bad@ contains irritating contaminants such as pollen or other allergens@ or contains dangerous chemicals at concentrations that may cause health effects. Unacceptable indoor air can have one@ two@ or all three of these characteristics. It may have odors only but not be unhealthy@ or it might be air that seems fine but contains dangerous concentrations of toxic chemicals that cannot be sensed by the occupants. In summary@ the purpose states that the standard prescribes ventilation@ building envelope@ and other measures intended to provide residential indoor air quality (IAQ) that is safe and pleasant."