Urban-associated diseases: Candidate diseases, environmental risk factors, and a path forward

Background: Cities are home to over half the global population; that proportion is expected to rise to 70% by mid-century. The urban environment differs greatly from that in which humans evolved, with potentially important consequences for health. Rates for allergic, inflammatory and auto-immune diseases appear to rise with urbanization and be higher in the more urbanized nations of the world which has led some to suggest that cities promote the occurrence of these diseases. However, there are no syntheses outlining what urban-associated diseases are and what characteristics of cities promote their occurrence. Objectives: To synthesize the current understanding of urban-associated diseases, and discover the common, potentially modifiable features of cities that may be driving these associations. Methods: We focus on any diseases that have been associated with cities or are particularly prominent in today's urban societies. We draw on expertise across diverse health fields to examine the evidence for urban connections and drivers. Discussion: We found evidence for urban associations across allergic, auto-immune, inflammatory, lifestyle and infectious disease categories. Some conditions (e.g. obesity and diabetes) have complex relationships with cities that have been insufficiently explored. Other conditions (e.g. allergies and asthma) have more evidence demonstrating their relationship with cities and the mechanisms driving that association. Unsurprisingly, air pollution was the characteristic of cities most frequently associated with disease. Other identified urban risk factors are not as widely known: altered microbial exposure and a disconnect from environmental microbiomes, vitamin D deficiency, noise and light pollution, and a transient, over-crowded, impoverished population. However, many complexities and caveats to these relationships beg clarification; we highlight the current knowledge gaps and outline ways to fill those gaps. Identifying urban-associated diseases and their drivers will allow us to prepare for the urban-disease burden of the future and create healthy cities that mitigate that disease burden.