In order to get an insight into the current status and future trends of indoor air pollution in China, pollution characteristics of major indoor air pollutants, including formaldehyde, VOCs, PM, biological pollutants, and SVOCs, are introduced as follows.
Formaldehyde is the most common and the best-known indoor air pollutant. In homes, the most significant sources of formaldehyde are likely to be engineered wood products made by using adhesives that contain urea-formaldehyde (UF) resins. Engineered wood products made for indoor use include particleboard (used as subflooring and shelving and in cabinetry and furniture), hardwood plywood paneling (used for decorative wall coverings and used in cabinets and furniture), and medium-density fiberboard (used for drawer fronts, cabinets, and furniture tops).
Indoor formaldehyde pollution was very serious in the early 2000s in China due to the extensive use of engineered wood products with high formaldehyde emission in home decoration and refurbishment. A survey conducted in 2003 in six cities of China indicated that the percentage of recently renovated homes with indoor formaldehyde concentrations above the national standard (0.10 mg/m3) reached 82.3% (Table 1) .
In recent years, formaldehyde pollution has been effectively alleviated by forbidding the use of substandard engineered wood products in indoor decoration and refurbishment via implementing the mandatory national standard, GB 18580-2001 “Indoor decorating and refurbishing materials – Limit of formaldehyde emission of wood-based panels and finishing products.” Table 2 shows the passing rates of formaldehyde emission of medium-density fiberboard and blockboard on the market in different years . It can be seen that for both kinds of wood-based panels, the passing rates of formaldehyde emission in 2009–2011 were much higher than those in 2003 or 2004.
As the awareness of the importance of IAQ has been increasing in these years, public demand for healthy/green/nontoxic decorating and refurbishing materials is growing. Predictably, the indoor formaldehyde pollution will be further reduced or completely eliminated with the development of wood-based panels with lower or zero formaldehyde emission in the near future.
2.2 Volatile Organic Compounds
VOCs have become major indoor air pollutants in China since the prevalence of interior decoration and refurbishment. Indoor sources of VOCs mainly include solvent coatings for woodenware, interior architectural coatings, adhesives, wood-based furniture, carpets, and carpet cushions. Generally, various kinds of VOCs coexist in indoor environments, in concentrations decreasing with time after renovation.
Similar to formaldehyde, indoor pollution of VOCs was also very serious in the early 2000s in China. An investigation conducted from 2002 to 2004 on VOCs concentrations in 1,241 recently renovated residences in China showed that the average concentration of total volatile organic compounds (TVOC) was as high as 2.18 mg/m3 and benzene, toluene, and xylenes (BTX) were the primary VOCs pollutants in indoor air, with average concentrations of 124.04, 258.90, and 189.68 μg/m3, respectively (Table 3) .
The high concentrations of BTX in newly renovated buildings could be mainly attributed to the widespread use of BTX as solvents and diluents for coatings and adhesives used in interior decoration and refurbishment. Considering the high toxicity of benzene to human beings, the use of benzene has been forbidden since 2002 by implementing mandatory national standards, including GB 50325-2001 “Code for indoor environmental pollution control of civil building engineering,” GB 18581-2001 “Indoor decorating and refurbishing materials – Limit of harmful substances of solvent coatings for woodenware,” and GB 18583-2001 “Indoor decorating and refurbishing materials – Limit of harmful substances of adhesives.” Meanwhile, the use of toluene and xylenes has been limited although they are less toxic compared to benzene. Field survey data prove that implementation of these standards has indeed reduced indoor pollution of benzene; however, indoor pollution of toluene and xylenes is still serious (Table 4) . In order to further control the pollution of toluene and xylenes, the abovementioned national standards have been revised successively in recent years to strengthen the limits of toluene and xylenes.
Besides promoting the use of less toxic organic solvents, great efforts have also been made in China to develop coatings and adhesives with no organic solvents, such as water-based coatings and adhesives, powder coatings, radiation curable coatings, and hot melt adhesives. Increasing use of these environment-friendly products in interior decoration and refurbishment would greatly reduce the indoor pollution of VOCs.
2.3 Particulate Matter
Compared with gaseous pollutants (formaldehyde, VOCs), indoor particulate matter (PM) has gathered much less concern in China, although inhalable PM (PM10, particles smaller than or equal to 10 μm in aerodynamic diameter) is included in the “Indoor air quality standard” (GB/T 18883-2002) as one of the 13 controlled chemical pollutants.
In China, indoor PM mainly originates from outdoor sources such as fuel burning (mainly coal), vehicle emissions, and transformation of gaseous emissions in the atmosphere . According to the air quality monitoring results of major cities in China since 2000, PM10 has been the most important primary pollutant (among PM10, SO2, NO2, CO, and O3) in ambient air . Figure 1 illustrates the annual average concentrations of PM10 in 31 cities in China for the years 2006, 2008, and 2010 [14–16]. It can be seen that the overall PM10 pollution was still serious in 2010 although an alleviation trend was observed for most cities. In 2006, 2008, and 2010, 19, 14, and 14 monitored cities had annual average concentrations of outdoor PM10 exceeding the limit value (0.10 mg/m3, annual average) stipulated by GB 3095-1996 “Ambient air quality standard” (Fig. 1).
On the other hand, the frequent occurrence of haze in many cities (especially mega cities such as Beijing, Shanghai, and Guangzhou) of China since the autumn of 2011 indicates that the concentration of fine particles (PM2.5, particles smaller than or equal to 2.5 μm in aerodynamic diameter) in ambient air has been increasing in recent years although the PM10 concentration tends to decrease. It is well known that fine particles are more hazardous to human health than coarse particles since they can travel deep into the lungs, enter the bloodstream, and penetrate into cells. In the newly revised “Ambient air quality standard” (GB 3095-2012), PM2.5 has been included as another common pollutant besides PM10, SO2, NO2, CO, and O3.
Since outdoor PM can migrate into indoor environments through fissures and cracks in the building structures, high concentration of PM in ambient air can not only deteriorate the quality of outdoor air but also negatively influence the quality of indoor air. Shi et al.  investigated the concentrations of PM in indoor and outdoor air of 541 homes in Taiyuan (the capital of Shanxi province) during both heating and non-heating periods of 2004–2006. Results showed that the PM concentration in outdoor air was always higher than that in indoor air and both the indoor and outdoor concentrations of PM during the heating period were higher than those during the non-heating period (Table 5). Correlation analysis proved that the outdoor PM pollution contributed greatly to the indoor PM pollution, especially during the heating period.
In addition to outdoor sources, indoor PM can also originate from indoor sources such as fuel-burning, cooking, smoking, and sweeping activities. For modern buildings with low ventilation rates, the presence of indoor sources may result in higher PM concentrations in indoor environments than in outdoor air (Table 6) .
In view of the severe outdoor pollution of PM, the significant influence of outdoor pollution on indoor environments as well as the serious health risks from PM exposure, strengthening research on indoor PM pollution and its control, has become imperative in China.
2.4 Biological Pollutants
Biological pollutants in indoor environments mainly include bacteria (including endotoxins from bacteria), fungi (including spores and cell fragments of fungi), viruses, dust mites, and animal dandruff. These pollutants exist in the air mainly as bioaerosols (biological particles). Major indoor sources of biological pollutants at residential homes include human occupants, pets, house dust, organic waste, as well as the heating, ventilation, and air-conditioning (HVAC) system [19–23]. Adverse health effects/diseases related to biological pollutants exposure can be divided into two categories: infectious diseases such as influenza, viral pneumonia (e.g., severe acute respiratory syndrome, SARS) and bacterial pneumonia (e.g., Legionnaires’ disease) and allergic diseases such as allergic asthma, allergic rhinitis, and allergic alveolitis .
Although studies on concentrations and health effects of indoor biological pollutants started in the 1950s in China [25–31], control of indoor biological pollution had not obtained adequate attention until the outbreak of SARS in 2003. Since bad-designed ventilation systems were considered to have played important roles in the rapid and widespread dissemination of the SARS virus, air-conditioning and ventilation systems have become the focus of concern in the control of indoor biological pollution.
In 2004, a national inspection on central air-conditioning and ventilation systems in 937 public places, including hotels, restaurants, shopping malls, and supermarkets, was performed by the Ministry of Health of the People’s Republic of China. The report showed that around half of the samples were heavily polluted with high concentrations of dust, bacteria, and fungi and the passing rate was only 6%. The maximum concentrations of dust, bacteria, and fungi on the inner surface of air ducts reached 486 g/m2, 277 × 104 cfu/(g dust), and 480 × 104 cfu/(g dust), respectively . During the use of air-conditioning, the dust containing bacteria and fungi can be transferred into indoor environments by the airstream, causing serious biological pollution and then adverse health effects.
In order to prevent and control the pollution caused by air-conditioning and ventilation systems, the Ministry of Health of the People’s Republic of China issued a comprehensive hygiene management approach and three supporting hygienic norms (hygienic norm, hygienic assessment norm, and cleaning norm) for central air-conditioning and ventilation systems in public places, which came into effect on March 1, 2006. Tables 7 and 8 present some survey data on hygienic conditions of central air-conditioning and ventilation systems in public places of China after implementing the management approach and hygienic norms [33–45]. It can be seen that the overall concentration-passing rates of typical pollutants both on the inner surface of air ducts (Table 7) and in the supply air (Table 8) have been significantly improved in recent years, indicating an effective control of biological pollution related with central air-conditioning and ventilation systems.
Compared with public places, however, much less concern has been given to control of biological pollution in residences, which should also be addressed in the future to protect residents from hazards of ubiquitous biological pollutants.
2.5 Semi-volatile Organic Compounds
SVOCs are organic compounds with boiling points in the range of 240–400°C . They are considered as new-type chemical pollutants in indoor air and have become a hot research topic in the field of indoor environment and health in recent years. Major sources of indoor SVOCs include materials and products containing plasticizers (additives in plastics to enhance their flexibility and extensibility) and flame retardants (additives in materials to reduce their combustibility), household pesticides, and human activities such as smoking, incense burning, and cooking .
China produces and consumes the largest amounts of plasticizers in the world. As shown in Fig. 2, 25% of plasticizers in the world were consumed by China in 2006 [47, 48]. Phthalates are produced and consumed in the largest amounts among all plasticizers, with diethylhexyl phthalate (DEHP) and dibutyl phthalate (DBP) being the two main types [47, 48].
The production and consumption of flame retardants are also very high in China. In 2006, China produced 270-kt flame retardants, 66.6% of which were halogen-based (chlorinated and brominated) (Fig. 3) . The largest production and consumption among halogen-based flame retardants belong to decabromodiphenyl ether (DecaBDE) due to its lowest price and highest performance .
China also produces and consumes great amounts of pesticides, the main active ingredients of which are SVOCs . Besides, field measurements show that smoking, coal combustion, and Chinese cooking produce large quantities of polycyclic aromatic hydrocarbons (PAHs) [49–51].
Epidemiological and toxicological studies have proved that SVOCs exposure can cause serious harm to human health, including harming the endocrine and reproductive systems [52–55]. In recent years in China, indoor SVOCs pollution has gathered escalating concern from the researchers. Several studies on pollution and exposure levels of indoor SVOCs have been conducted, and the results show that indoor SVOCs pollution might be very serious in China [56–60]. In order to prevent and control this new type of pollution, more concern from the government as well as the public is required and more systematic and in-depth research should be carried out.