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Nestled in northwestern Cambodia's Siem Reap Province, Siem Reap serves as the gateway to the UNESCO World Heritage site of Angkor Archaeological Park, positioning it at the heart of Cambodia's tourism economy. The city lies on the flat alluvial plains of the Tonlé Sap Basin, approximately 15 kilometers south of the vast Tonlé Sap Lake—Southeast Asia's largest freshwater lake—and about 320 kilometers northwest of Phnom Penh. At an elevation of around 18 meters above sea level, Siem Reap's low-lying terrain contributes to stagnant air conditions that trap pollutants, particularly during the dry season. The urban area blends a compact central district with sprawling peri-urban and rural fringes, creating a distinct urban–rural gradient where agricultural activities interface directly with tourism infrastructure. Surrounding landscapes consist of rice paddies, scrubland, and remnants of dry deciduous forests, with the Kulen Mountains to the northeast providing some topographic variation. This geography profoundly influences air quality: proximity to Tonlé Sap Lake offers minimal moderating effects on pollution, while the flat plains allow agricultural burning from nearby farms to drift unimpeded into the city. The lack of significant industrial belts means pollution stems predominantly from local sources—vehicle emissions from tuk-tuks and tour buses, road dust on unpaved routes, and construction linked to tourism development—all exacerbated by the region's climatic patterns.

Siem Reap's tropical wet-and-dry climate drives a stark seasonal cycle in air quality, with pollution peaking dramatically from February through April. During this dry season, northeast monsoon winds weaken, leading to stagnant atmospheric conditions, temperature inversions, and minimal rainfall that allow pollutants like vehicle exhaust, construction dust, and agricultural burning residues to accumulate. March and April are particularly hazardous due to extreme heat and drought, which intensify biomass and waste burning, while low humidity raises fine particulate matter levels. Sensitive groups, including children, the elderly, and those with respiratory conditions, should avoid prolonged outdoor activities during these months, especially in peak afternoon hours. From May to October, the southwest monsoon brings heavy rains that cleanse the air, with June to September offering the lowest pollution levels and favourable conditions for tourism, though flooding can pose other risks. The transitional months of November to January see moderate air quality as humidity drops and winds shift, but occasional haze from early agricultural fires may occur. Year-round, domestic biomass cooking contributes a persistent baseline of indoor pollution, underscoring the need for ventilation and air purifiers in homes. For visitors, planning trips during the wet season or early dry period (November–January) optimises outdoor experiences while minimising health impacts from poor air quality.