THE WORLD HEALTH ORGANIZATION GUIDELINES

EM The Urban Environment THE WORLD HEALTH ORGANIZATION GUIDELINES FOR AIR QUALITY Part 1: Exposure-Response Relationships and Air Quality Guidelines Two recently updated WHO documents, Air Quality Guidelines for Europe and Guidelines for Air Quality, provide a basis for protecting public health from the adverse effects of environmental pollutants. This first of a two-part article highlights the guideline documents major topics and considers their applicability throughout the world. by Dietrich Schwela A mbient and indoor air pollution is a major environmental health problem in developed and developing countries around the world. On a global scale, increasing amounts of potentially harmful gases and particles are being emitted into both ambient and indoor environments, resulting in damage to human health and the environment. The risks posed to human health by air pollution have been evaluated by the World Health Organization (WHO) since the 1950s, and guideline values for limiting air pollutant concentrations were first derived in 1972.1 In 1987, the WHO Regional Office for Europe (EURO) published the Air Quality Guidelines for Europe.2 Since 1993, WHO/EURO has revised and updated these guidelines in several meetings.3 During this time, the International Programme on Chemical Safety has continued the Environmental Health Criteria (EHC) series, assessing the health risks of more than 120 chemical compounds and mixtures between 1987 and 1999. Additional compounds were analyzed in the Concise International Chemical Assessment Documents of the Inter-Organization Programme for the Sound Management of Chemicals. July 2000 EM 29

EM The Urban Environment The Air Quality Guidelines for Europe is based on evidence from epidemiological and toxicological literature published in Europe and North America. 2 It does not consider the effects of exposure to the different ambient air pollutant concentrations in developing countries, or the different conditions there. However, the document has been used extensively throughout the world. In view of the different conditions in developing countries, the literal application of the revised and updated Air Quality Guidelines for Europe can be misleading. Factors such as high and low temperatures, humidity, altitude, background pollutant concentrations, and the population s nutritional status can all influence human health conditions at the same time when the population is exposed to air pollution. To make the Air Quality Guidelines for Europe globally applicable, a task force group meeting was convened at WHO headquarters in December 1997. The Air Quality Guidelines for Europe 3 was extended to provide global coverage and applicability, and the issues of air quality assessment and control were addressed in more detail. 4 Moreover, the guidance given in the publications of the EHC series was also incorporated in the new document titled Guidelines for Air Quality, to substantially extend the number of compounds for which guidance can be provided. 4 These documents (hereafter referred to as Guidelines ) provide a basis for protecting public health from the adverse effects of environmental pollutants by providing background information and guidance to governments for making risk management decisions, particularly in setting standards. The documents also help governments carry out local action plans for rational air quality management. The guidelines therefore must be seen in the context of air quality management. These two documents have been prepared as a practical response to the need for air pollution abatement at the local level, and for improved legislation, management, and guidance at the national and regional levels. Continuing efforts 30 EM July 2000 will be made to improve their content and structure. This article highlights the main topics of the Guidelines. For identical compounds in both documents, the guideline values and functional relationships between exposure and response are based on the original Air Quality Guidelines for Europe, and are numerically the same. The Guidelines emphasize the framework of air quality management, the application of the guidelines in different environments on a global scale, the relationship of guidelines to standards, and prioritysetting in the managing of ambient and indoor air pollution. Exposure to Air Pollutants The Guidelines is focused on those gaseous pollutants gases and vapors and suspended particulate matter (SPM) dusts, fumes, mists, and smokes that have been accepted as posing a threat to human health. The relative health threat of different pollutant gases and particles varies with their concentrations over time and distance, implying that the effects of air pollutants on health may vary from country to country. Consequently, careful monitoring of the concentrations of polluting gases and particles, as well as of the particle size distribution and composition, is needed before an acceptable estimate of the effects can be produced. The picture is further complicated because some pollutant combinations act in an additive manner, while some may act synergistically. Many factors can account for the substantial differences between the concentrations of pollutants measured at central locations and those in the normal breathing zone of residents of the community. Many of these factors can be modeled; such models have been used for estimating dose distributions associated with ambient air concentrations. Local concentrations of air pollutants depend upon the strength of their sources and the efficiency of their dispersion. Day-to-day variations in concentrations are more affected by meteorological conditions than by changes in source strengths. Wind is of key importance in dispersing air pollutants, and for ground-level sources, pollutant concentrations are inversely related to wind speed. Turbulence is also important: a rough terrain, such as that produced by buildings, tends to lead to increased turbulence and better dispersion of pollutants. An individual s total daily exposure to air pollution is the sum of the separate contacts to air pollution experienced by that person as he/she passes through a series of environments (also called microenvironments) during the course of the day (e.g., at home, while commuting, in the streets). Exposures in each of these environments can be estimated as the product of the concentration of the pollutant in question and the time spent in the environment. The Guidelines place some emphasis on epidemiological data. Epidemiological studies are sometimes preferable to controlled exposure studies in that they provide information on responses in populations and on the effects of real exposures to pollutants and pollutant mixtures. However, the results of epidemiological studies are less easy to use than the results of controlled exposure studies in defining guidelines. For both particles and O 3, a linear relationship was assumed in the definitions of the exposure-response relationships included in the revised guidelines. For O 3, however, this assumption is questionable, as the relationship at higher concentrations may be concave upward. 4 For particles, linear relationships are valid below concentrations of 100 200 µg/m 3. Extrapolation beyond the available data is dangerous, as there is evidence to suggest that the exposure-response relationship may become less steep as ambient levels of particles rise. These are important points to consider if the Guidelines are to be used in countries with levels of pollution different from the range covered by the Guidelines.

Health-based Guidelines The new Guidelines are based on epidemiological time-series studies that emerged in the late 1980s and 1990s, developed first in the United States and later in Europe and other areas. In essence, the time-series approach takes the day as the unit of analysis and relates the daily occurrence of events such as deaths or hospital admissions to daily average pollutant concentrations while taking careful account of confounding factors such as season, temperature, and day of the week. These studies applied powerful statistical techniques and have produced coefficients that relate daily average pollutant concentrations to human health effects. Associations have been demonstrated between daily average concentrations of particles, O 3, SO 2, airborne acidity, NO 2, and CO. Although the associations for each of these pollutants to human health levels were not significant in all studies, when taking the body of evidence as a whole, the consistency is striking. Many researchers have agreed that for particles and O 3, the studies provide no indication of any threshold of effect. Key air pollutants, also termed classical air pollutants SO 2, NO 2, CO, O 3, SPM, and Pb are briefly described below with respect to their health risk evaluation. SO 2 Several time-series studies have observed associations between SO 2 and daily mortality and morbidity. In particular, singlepollutant correlations sometimes disappeared when other pollutants, especially SPM, were included. In crosssectional studies with asthmatics, a particularly susceptible group, significant, non-threshold relationships were found between SO 2 exposure and decrements of the 1-sec forced expiratory volume. NO 2 Weak associations are indicated between short-term NO 2 exposure due to gas cooking and respiratory symptoms and decrement in lung function parameters in children. This association is not consistently found in exposed women. A number of outdoor studies indicate that children with long-term exposure exhibit increased respiratory symptoms, decreases in lung function, and increased incidences of chronic cough, bronchitis, and conjunctivitis. Other long-term studies provide little evidence of such effects of NO 2 in adults. There is still not enough evidence from epidemiological studies for a causal relationship between NO 2 and observed health effects. CO Potential CO health effects include hypoxia, neurological deficits, neurobehavioral changes, and increases in daily mortality and hospital admissions for cardiovascular diseases. Several studies showed small, statistically significant relationships between CO and daily mortality. Some studies indicate that the association between ambient CO and mortality and hospital admissions due to cardiovascular diseases persists even at very low CO levels, indicating no threshold for the onset of these effects. It is possible that ambient CO may have more serious health consequences than the carboxyhämoglobin (COHb) formation, and at lower levels than those mediated through elevated COHb levels. A question still exists of whether the demonstrated associations are causal or if CO may act as a proxy for PM. O 3 Short-term acute effects of O 3 concentrations include pulmonary function decrements, increased airway responsiveness and inflammation, aggravation of pre-existing respiratory diseases such as asthma, increases in daily hospital admissions and emergency room visits for respiratory distress, and excess mortality. Exposure-response relationships appear to be nonlinear for the associations between O 3 concentration and the 1-sec forced expiratory volume, inflammatory changes, and hospital admissions changes, respectively. A linear relationship was established for the association between the percent change in symptom exacerbation among adults and asthmatics. In studies on the relationships between O 3 exposure and daily mortality and hospital admissions for respiratory diseases, single-pollutant associations between O 3 and these health effects remained statistically significant even in multi-pollutant models. SPM An extensive body of experimental and epidemiological literature demonstrates that significant associations between SPM concentrations and the rates of mortality and morbidity exist in the human population. Human health effects of SPM depend on particle size and concentration and can respond to daily fluctuations in PM 10 or PM 2.5 levels. They include acute effects such as increased daily mortality, increased hospital admission rates for exacerbation of respiratory disease, fluctuations in the prevalence of bronchodilator use and cough, and peak flow reductions. Relationships between PM 10 or PM 2.5 exposure and acute health effects appear to be linear at concentrations below 100 µg/m 3. A much smaller number of studies refer to the long-term effects of SPM with respect to mortality and respiratory morbidity. The current time-series epidemiological studies do not indicate a threshold below which no effects occur. Rather, they suggest that even at low levels of SPM, short-term exposure is associated with daily mortality, daily hospital admissions, exacerbation of respiratory symptoms, bronchodilator use, cough, and peak expiratory flow. Pb The potential effects of lead in adults and children include negative encephalopathic signs and symptoms, central nervous system problems, cognitive effects, increased blood pressure, and a reduction in levels of child intelligence. While the lowest-observed levels have July 2000 EM 31

EM The Urban Environment been established for most of these effects, the question of increased blood pressure is still controversial. The potential for low levels of lead exposure to cause mental deficits is still being debated. The new air quality guidelines for the gaseous compounds and lead are Table 1. WHO guideline values for the classical air pollutants. 3,4 presented in Table 1. For PM, the Guidelines for Air Quality do not present guideline values such as those for the gaseous compounds, but rather present exposure-response relationships. Two examples of such relationships are provided in Figures 1 and 2. Further Compound Guideline Averaging Compound Guideline Averaging Value (µg/m 3 ) Time Value (µg/m 3 ) Time CO 100,000 15 min NO 2 200 1 hr 60,000 30 min 40 1 yr 30,000 1 hr 10,000 8 hr Pb 0.5 1 yr SO 2 500 10 min 125 24 hr O 3 120 8 hr 50 1 yr examples and their interpretation are provided in the Guidelines. In Table 2, the guideline values, tolerable concentrations, or tolerable daily intakes for compounds with non-carcinogenic endpoints are presented. In Table 3, the guidelines for air pollutants with carcinogenic endpoints are presented in the form of unit risks (for the definition of the unit risk, see ref 2) together with the classification of the International Agency for Cancer Research (IARC). The unit risks classify carcinogenic compounds according to their potential to produce cancer in a population during an average lifetime exposure. They can, therefore, serve to set priorities in policy decisions. The Guidelines also present unit risks for several non-heterocyclic polycyclic aromatic hydrocarbons (Table 4), Table 2. Guidelines for air quality: compounds with non-carcinogenic health endpoints. Compound Guideline Value (GV) Averaging Time Compound GV or TC (µg/m 3 ) Averaging Time or Tolerable Concentration (TC) (µg/m 3 ) Acetaldehyde 4 2000 (TC) 24 hr Formaldehyde 3 100 (GV) 30 min 50 (TC) 1 yr Hydrogen sulfide 2 7 (GV) 30 min 150 (GV) 24 hr Acrolein 4 50 (GV) 30 min Manganese 3 0.15 (GV) 1 yr Acrylic acid 4 54 (GV) 1 yr Mercury, inorganic 3 1 (GV) 1 yr 2-butoxyethanol 4 13100 (TC) 1 week Methyl methacrylate 4 200 (TC) 1 yr Cadmium 3 5 x 10 3 (GV) 1 yr Monochlorobenzene 4 71 (TC) 1 yr Carbon disulfide 2 100 (GV) 24 hr Styrene 3 260 (GV) 1 week 20 (GV) 30 min 7 (GV) 2 30 min Carbon tetrachloride 4 6.1 (TC) 1 yr Tetrachloroethylene 3 250 (GV) 24 hr 8000 (GV) 30 min 1,4- dichlorobenzene 4 134 (TC) 1 yr Toluene 3 260 (GV) 1 week 1000 (GV) 2 30 min Dichloromethane 3 3000 (GV) 24 hr 1,3,5- trichlorobenzene 4 36 (TC) 1 yr Diesel exhaust 4 5.6 (GV) a 1 yr 1,2,4- trichlorobenzene 4 8 (TC) 1 yr 2.3 (GV) a 1 yr Ethylbenzene 4 22,000 (GV) 1 yr Vanadium 2 1 (GV) 24 hr Fluorides 3 1 (GV) 1 yr Xylenes 4 4800 (GV) 24 hr 870 (GV) 1 yr Tolerable or Time (over lifetime) TDI (TEQ/kg bw d) c Time (over lifetime) Average Daily Intake (TDI or ADI) (µg/kg bw d) b Chloroform 4 15 (TDI) 24 hr Dioxin-like compounds 6 1 4 (TDI) 24 hr Cresol 4 170 (ADI) 24 hr Di-n-butyl phthalate 4 66 (ADI) 24 hr Notes: a For diesel exhaust, two approaches were applied, based on a NOAEL of 0.41 mg/m 3 in rats. The corresponding levels were converted to a continuous exposure scenario. b µg/kg bw d = microgram per kg body weight and day. c TEQ/kg bw d = toxicity equivalents per kg body weight and day. 32 EM July 2000

Percent Increase ± ± (sulfates) PM Concentration [ µg m -3 ] Figure 1. Increase in daily mortality as a function of PM concentration. has a very significant effect on health, whereas humidity is unlikely to have a significant effect on the toxicity of gaseous pollutants. The age structure of populations differs markedly from country to country. Older people tend to show increased susceptibility to air pollution. Very young children may also be at increased risk. People with a poor standard of living suffer from nutritional deficiencies, infectious disease due to poor sanitation and overcrowding, and inferior medical care. Each of these factors may render individuals more susceptible to the effects of air pollution. Diseases that produce narrowing of the airways, reduction in the area of the gasexchange surface of the lung, and increased alteration of inhalation-perfusion ratios likely increase the subject s susceptibility to the adverse effects of a range of air pollutants. Percent Increase PM Concentration [ µg m -3 ] Figure 2. Percent change in hospital admissions assigned to PM 10, PM 2.5, and sulfates. which were derived from the potency of the compounds relative to benzo[a]pyrene (BaP). 5 AIR POLLUTANT CONCENTRATIONS AND FACTORS AFFECTING SUSCEPTIBILITY Altitude, temperature, and humidity vary significantly across the globe. At increased altitude, the partial pressure of oxygen falls and inhalation increases to compensate. Increased inhalation leads to an increased intake of airborne particles. However, for gaseous pollutants no increase in adverse health effects from those experienced at sea level would be expected. Temperature ± (sulfates) CLASSICAL AIR POLLUTANTS: THE APPLICABILITY OF EUROPEAN GUIDELINES ON A WORLDWIDE SCALE In the derivation of the WHO Air Quality Guidelines for Europe, assumptions were made for some airborne polluting compounds that may not be applicable in parts of the world. For example, the importance of different exposure routes for some pollutants may vary from country to country. It should be understood that if such factors were to be taken into account, different guidelines could be derived. The assessment of carcinogenic compounds using the unit risk model is also dependent upon considerations of the comparative importance of different routes of exposure. It is important that regulatory authorities evaluate the applicability of the WHO Guidelines for Air Quality as a basis for setting local guidelines or standards. These remarks apply to the guidelines of all compounds considered, as studies from developing countries are relatively scarce. Part 2 of this article, which will appear in the next installment of The Urban Environment, will examine the role of the WHO guidelines in air quality management. Table 3. Guidelines for air pollutants with carcinogenic health endpoints. Compound Unit Risk (µg/m 3 ) 1 IARC Classification Compound Unit Risk (µg/m 3 ) 1 IARC Classification Acetaldehyde 4 (1.5 9) x 10 7 2B 1,2-dichloro-ethane 4 (0.5 2.8) x 10 6 2B Acrylonitrile 2 2 x 10 5 2A Diesel exhaust 4 (1.6 7.1) x 10 5 2A Arsenic 3 1.5 x 10 3 1 ETS 3 10 3 1 Benzene 3 (4.4 7.5) x 10 6 1 Nickel 3 3.8 x 10 4 1 Benzo[a]pyrene 3 8.7 x 10 2 1 PAH (BaP) 3 8.7 x 10 2 1 Bis(chloromethyl)ether 4 8.3 x 10 3 1 1,1,2,2-tetrachloro-ethane 4 (0.6 3.0) x 10 6 3 Chloroform 4 4.2 x 10 7 2B Trichloro-ethylene 3 4.3 x 10 7 2A Chromium VI 3 (1.1 13) x 10 2 1 Vinychloride 2 1 x 10 6 1 July 2000 EM 33

EM The Urban Environment Table 4. Estimate of unit risks for several polycyclic aromatic hydrocarbons. 4 Compound Relative Potency Unit risk (µg/m 3 ) 1 Range Compared to BaP Anthanthrene 0.28 0.32 (2.4 2.8) x 10 2 Benz[a]anthracene 0.014 0.145 (1.2 13) x 10 4 Benzo[a]pyrene 1 8.7 x 10 2 Benzo[b]fluoranthene 0.1 0.141 (0.87 1.2) x 10 2 Benzo[j]fluoranthene 0.045 0.1 (0.4 0.87) x 10 2 Benzo[k]fluoranthene 0.01 0.1 (8.7 87) x 10 4 Chrysene 0.001 0.1 (8.7 870) x 10 5 Cyclopenta[cd]pyrene 0.012 0.1 (1 8.7) x 10 3 Dibenzo[a,e]pyrene 1 8.7 x 10 2 Dibenz[a,c]anthracene 0.1 8.7 x 10 3 Dibenz[a,h]anthracene 0.89 5 (7.7 43.5) x 10 2 Dibenzo[a,l]pyrene 100 8.7 x 10 0 Dibenzo[a,e]fluoranthene 1 8.7 x 10 2 Dibenzo[a,h]pyrene 1 1.2 (8.7 10.4) x 10 2 Dibenzo[a,i]pyrene 0.1 8.7 x 10 3 Fluoranthene 0.001 0.01 (8.7 87) x 10 5 Indeno[1,2,3,-cd]pyrene 0.067 0.232 (5.8 20.2) x 10 3 REFERENCES 1. Air Quality Criteria and Guides for Urban Air Pollutants; Report of a WHO Expert Committee, Technical Report Series No. 506; World Health Organization: Geneva, 1972. 2. Air Quality Guidelines for Europe; WHO Regional Publications, European Series No. 23; Regional Office for Europe, World Health Organization: Copenhagen, Denmark, 1987. 3. Air Quality Guidelines for Europe; WHO Regional Publications, European Series; Regional Office for Europe, World Health Organization, Copenhagen, 1999. Available at http://www.who.dk. 4. Guidelines for Air Quality; World Health Organization Geneva, 1999. Available at http://www.who.int/peh. 5. Selected Non-Heterocyclic Polycyclic Aromatic Hydrocarbons. In Environmental Health Criteria 202; World Health Organization: Geneva, 1998. 6. Assessment of the Health Risk of Dioxins: Re-Evaluation of the Tolerable Daily Intake (TDI). WHO Consultation, May 25 29, 1998, Geneva, Switzerland, WHO European Centre for Environment and Health, International Programme on Chemical Safety, World Health Organization: Geneva, 1998. Available at http://www.who.int/pcs/pubs/dioxin-exec-sum/exe-sumfinal.html. About the Author Dietrich Schwela is an Air Pollution Scientist with the Department of Protection of the Human Environment, Occupational and Environmental Health Programme, World Health Organization, 20 Avenue Appia, CH 1211, Geneva 27, Switzerland; phone: +41 22 791 4261; e-mail: schwelad@who.int. 34 EM July 2000