BackgroundAlthough socio-environmental factors which may affect dementia have widely been studied, the mortality of dementia and environmental relationships have seldom been discussed. Method: A register-based study was conducted to evaluate the relationships of four individual-level social measures, two community-level social measures, six short-term (temporal-varying) environmental measures, and four long-term (spatial-varying) environmental measures with dementia mortality in a high-density Asian city (Hong Kong), for the following decedents: 1) all ages, 2) “old-old”: age >= 85, 3) “mid-old”: aged 75 - 84, and 4) “young-old”: aged 65 – 74. ResultsThis study identified 5438 deaths (3771 old-old; 1439 mid-old; 228 young-old) from dementia out of 228,600 all-cause deaths among older adults in Hong Kong between 2007 and 2014. Generally, regional air pollution, being unmarried or female, older age, and daily O3 were associated with higher dementia mortality, while more urban compactness and greenness were linked to lower dementia mortality. Specifically, being unmarried and age effect was linked to higher dementia mortality among the “old-old”, “mid-old” and “young-old”. Regional air pollution was linked to increased dementia mortality, while urban compactness and greenness were associated with lower dementia mortality among the “old-old” and “mid-old”. Higher daily O3 had higher dementia mortality, while districts with a greater percentage of residents whose native language is not Cantonese was linked to lower dementia mortality among the “old-old”. Economic inactivity was associated with increased dementia mortality among the “young-old”. Gender effect varied by age.ConclusionThe differing strengths of association of various factors with dementia mortality among different age groups implies the need for a comprehensive framework for community health planning. In particular, strategies for air quality control, usage of greenspace and social space, and activity engagement to reduce vulnerability at all ages are warranted.