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Sandakan occupies a distinctive coastal position on the northeastern edge of Sabah, Malaysia, nestled along the Sulu Sea with its urban core hugging Sandakan Bay. This port city, historically known as 'Little Hong Kong,' sits on relatively flat terrain at sea level, though it is flanked by the steep, forested ridges of the Sandakan Peninsula to the north and east, creating a natural amphitheater that can trap air pollutants. The city's geography is defined by its proximity to water—with the bay to the west and the Celebes Sea accessible via channels—and its role as a gateway to Borneo's interior, featuring a mix of urban development, oil palm plantations, and remnant rainforests. This setting influences air quality through a combination of maritime breezes that typically disperse local emissions and occasional stagnation when land breezes prevail, especially during nighttime. The urban-rural gradient is sharp: dense commercial and residential zones along the coast transition rapidly into agricultural belts, notably oil palm estates, which are significant sources of biomass burning and particulate matter during land-clearing activities. Sandakan's position within Southeast Asia's equatorial belt means it experiences consistent humidity and rainfall, which generally aid in pollutant washout, but its coastal industrial activities, including shipping and processing of agricultural products, contribute to baseline pollution levels. The surrounding landscape, with hills to the east, can lead to temperature inversions that trap pollutants close to the ground, exacerbating poor air quality episodes, particularly when combined with urban emissions from vehicles and small-scale industries.

Sandakan's air quality story is shaped by its equatorial monsoon climate, with distinct wet and dry seasons rather than four traditional seasons, leading to predictable pollution patterns tied to rainfall and wind shifts. During the northeast monsoon from November to March, heavy rainfall and persistent winds from the South China Sea sweep across the region, effectively cleansing the air and resulting in the lowest pollution levels; this period is ideal for outdoor activities, with sensitive groups such as asthmatics and the elderly experiencing minimal respiratory risks. Conversely, the southwest monsoon from May to September brings drier conditions and variable winds, often from inland areas, which can transport smoke from agricultural fires in neighboring regions like Kalimantan, leading to sporadic haze episodes that peak around July to September—these months should be avoided for strenuous outdoor exertion. Inter-monsoon periods in April and October see transitional weather with lighter winds and higher temperatures, increasing the likelihood of temperature inversions that trap local pollutants from urban sources like traffic and industrial operations, causing moderate air quality degradation. Fog is rare, but high humidity can combine with particulate matter to reduce visibility and exacerbate health issues. Sensitive groups should monitor local air quality reports during dry spells, limit outdoor exposure on hazy days, and use air purifiers indoors when pollution levels rise, as these conditions can trigger respiratory and cardiovascular problems. Overall, the wettest months offer the cleanest air, while the drier intervals require vigilance due to both local and transboundary pollution influences.