respectively。 The air can be purified with filtration and air cleaning units or equipment that is installed in the system providing ventilation, or as stand-alone solutions displaced in different lo- cations in buildings (e。g。, [35,69,70]。 The air can also be purified by using building and finishing materials that are especially manu- factured to act as the air cleaners (e。g。, [22,25,51]。

Although it may be expected that air purification systems would benefit IAQ, two major problems have been related with this technology (e。g。, [7,70,85,94]。 One problem is that the air cleaners may not be as effective as they are claimed to be, or they may selectively remove only some pollutants and be ineffective for the others。 The other problem is that some air cleaners during the process of cleaning will generate unwanted products, some even more harmful than the pollutants that are removed。 Additionally, some air cleaning and filtration techniques can become the source of pollution if not properly serviced or maintained (e。g。,  [11,48]。

Flushing or enhanced ventilation is usually exercised at night (so-called nighttime ventilation) or during the off-hours when the building is unoccupied。 It significantly increases outdoor air supply rates and uses the cooling power of the ambient air to remove the heat accumulated in the building。 However, this process can also bring in outdoor pollutants that can accumulate indoors。 This is especially problematic in areas with high outdoor pollution and when the filtration and air cleaning systems on buildings cannot adequately remove pollutants from high air volumes。 Enhanced ventilation can also break the equilibrium near the surfaces of materials。 This can lead to release of pollutants, which are primarily emitted by materials (intrinsic emissions) and also those that adsorb on surfaces and are absorbed in the building  structures。

As another common example, outdoor air supply rates may be reduced in order to promote energy efficiency but without compensatory actions such as source control。 This can increase concentration of pollutants indoors and reduce IAQ, especially in existing buildings that undergo renovation and retrofit。 Tightening of building envelopes can also reduce outdoor air supply rates, which can reduce IAQ, if the volume of air that is infiltrating indoors is not brought back out by the ventilation system or if pollution sources are not concurrently reduced。 Tighter structures and reduced infiltration can, on the other hand, reduce the penetration of outdoor pollution indoors, which is important when outdoor levels are undesirably high。 Which of the two brings more benefits depends on the particular building and the  context。

A frequent feature of green building projects is to locate the building close to urban centers or public transport systems, which can earn credits for location or transport。 However, location of buildings close to dense urban areas or major transportation routes may result in poor outdoor air quality that can penetrate indoors, and reduce IAQ。 Green solutions for ventilation such as natural ventilation or hybrid (dual-mode) ventilation may result  in elevated exposures to outdoor pollution。 These solutions may not use filtration and air cleaning technologies to remove pollutants from outdoor air used for ventilation, and thus may impair IAQ especially in cases when outdoor pollution is   high。

In conclusion, some green practices may earn credits in green building certifications but result in poor IAQ。 These practices include, among others, the use and recycling of products that contain hazardous compounds, energy efficiency strategies that increase indoor pollutants, location of green buildings near urban and transportation emissions, and the use of natural ventilation in areas with elevated outdoor pollution。

1。6。 How can green products compromise the IAQ of a green building?