## 1 Introduction  
Bangkok, the capital city of Thailand with 10.7 million people [1], has been facing prolonged air quality challenges, including challenges resulting from annual average particulate matter (PM\({}_{10}\)) concentrations that exceed the WHO recommended guideline of 20 ug m\({}^{-3}\). In 2011, the daily mean concentrations of PM\({}_{10}\) in Bangkok ranged between 10.0 and 189.0 ug m\({}^{-3}\) at roadside sites and 7.4 and 131.5 ug m\({}^{-3}\) in background areas, with 2.7% and 0.2% of daily mean PM\({}_{10}\) concentrations at these sites exceeding the Thailand National Ambient Air Quality Standard (NAAQS) of 120 ug m\({}^{-3}\)[2]. This may represent a significant public health risk in relation to the WHO recommended guideline for a 24-h mean PM\({}_{10}\) of 50 ug m\({}^{-3}\)[3]. High ambient concentrations of PM\({}_{10}\) increase the potential for adverse health effects caused by toxic contaminants, particularly carcinogenic and mutagenic polycyclic aromatic hydrocarbons (PAHs). PAHs emitted from both natural (e.g., forest fires) and anthropogenic (e.g., fossil fuel combustion) sources are mostly bound to airborne particles (pPAHs) [4]. Carcinogenic pPAHs are mostly high-molecular-weight low-volatility compounds dominantly adsorbed onto particles [5]. pPAHs of health concern owing to their carcinogenic and teratogenic properties include: fluorene [FLU], phenanthrene [PHE], anthracene [ANT], fluoranthene [FLT], pyrene [PRY], benzopyrene [BP], benzo[a]pyrene [BaP], dibenz[a,h]anthracene [DBA], benza[a]anthracene [BaA], benzo[b]fluoranthene [BBF], benzo[k]fluoranthene [BKF], chrysene [CHR], and inden [1,2,3-cd]pyrene [IP] [6]. Studies on adverse health effects of pPAHs have been conducted in many countries [4; 7; 8; 9; 10; 11]. The carcinogenic effect of exposure to atmospheric pPAHs resulted in the development of tumors in the pharynx and larynx of exposed hamsters [12]. DNA damage in Chinese hamster lung fibroblasts was associated with high-molecular-weight PAHs [13]. pPAHs pose a human health risk concern as they are distributed in the respirable particle size range; thus, exposure by inhalation is unavoidable [14; 15]. The European Commission (EC) currently uses benzo[a]pyrene as a marker for carcinogenic PAHs in ambient air and set a target value of 1 ng m\({}^{-3}\)[16], whereas the UK adopted a more stringent standard for an annual average concentration of benzo[a]pyrene of 0.25 ng m\({}^{-3}\)[17]. Published studies on pPAHs have evaluated the health risk according to toxicity with reference to benzo[a]pyrene [18; 19; 20]. The lifetime lung cancer risk is estimated to reflect the potential health effects resulting from exposure to atmospheric PAHs. The estimation of lung cancer risk caused by PAHs can be evaluated from exposure concentrations of individual PAHs and their carcinogenic potency. Carcinogenic risk assessment can be undertaken using benzo[a]pyrene as a surrogate for carcinogenic PAHs or by combining the toxicity of individual PAHs according to benzo[a]pyrene toxicity equivalent factors [18; 21]. Long-term exposure to high PM\({}_{10}\) and PM\({}_{10}\)-bound PAHs may pose a significant health risk, particularly in urban areas of the Bangkok Metropolitan Administration (BMA). Although ambient PM\({}_{10}\) has been regulated in Thailand, systematic pPAH measurement has not previously been undertaken. Published studies on the exposure of carcinogenic PAHs focused on highly polluted areas, and groups of exposed people and occupations, emphasize the need to manage these pollutants because of their significant health risk [20; 22]. PM\({}_{10}\) in ambient air is regulated under the Thailand NAAQS that specifies ambient PM\({}_{10}\) concentrations of 120 and 50 \(\mu\)g m\({}^{-3}\) for 24-h and annual averages [2].