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| Engineering control of any environment can increase or decrease ecological impacts. For example, HVAC (heating, ventilation, air conditioning) in any building is designed to control the flow of air inside the building and between the building and the atmosphere outside. Air filters help to improve air quality when applied to inside or outside air exchange, or both. Since no one filter is 100% effective, what remains is the actual risk to human health. Risk then is narrowed to qualities of indoor and outdoor air that are not controlled by HVAC and include chemical and organic compounds too small to be filtered or that require highly specialized filters that may not be available or in use.
A common argument is that to reduce health risk fresh air is required to dilute or minimize the impact of uncontrolled substances. For example, an outside air exchange of 20% leaves 80% indoor air being recycled. Thus, indoor risks known to arise from chemical outgassing that collect in all buildings are supposed to be controlled by HVAC intake and exchange with outside air. However this assumes that the outside air is of better quality. A problem arises in cities where smog collects in the outside air to the extent that health warnings are issued and sensitive people are advised to stay indoors. This advice is not consistant with the fact that building air quality depends on exchange with a better quality of outside air to reduce actual indoor health risk. Health risk increases indoors proportional to the increase of substances that are not filtered by HVAC control like new carpets, composite furniture, paint, cleaners, perfumes, etc. If the HVAC is on, health risk increases by adding smog. If the HVAC is off, air is not circulated, filtered, or exchanged, and harmful substances collect. Therefore, in order to reduce health risk, both external and internal measures of control must go beyond general HVAC and include selection and use of less toxic materials supplemented by plants that help to remove toxins from the air. Human health is one indicator of the ecological impact of environmental control. If control is to reduce health risk, then the measures must include, not exclude, appropriate choice of material and technology that works with or not against other eco-systems. When indicators of illness like MCS are present, measures of control must be changed to move towards help and away from harm. In this way relief for the sick also becomes prevention for the well. For the purpose of this study of PEC, the examples used to describe ecologic impacts are just a few of the many and complex relations between humans and the environment.
The word ecology was coined by the German zoologist Ernst Haeckel in 1869. Literally, the term ecology comes from the Greek words oikos, "the family household", and logy, "the study of".
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