Redefining public health in New York City.

“If you want to live longer and healthier than the average American, come to New York City”, pronounced New York City’s Mayor Michael Bloomberg as he released updated data on the city’s life expectancy last December. The numbers gave him reason to crow: from a nadir in 1990, when life expectancy in the city trailed the US average by 3 years, it had lengthened by 8 years to 80·6 years, surpassing the country. In the national context, the increase in New York City’s life expectancy stands out (fi gure). The Institute for Heath Metrics and Evaluation recently estimated the life expectancy for each of the USA’s 3147 independent cities and counties. Manhattan’s life expectancy rose 10 years between 1987 and 2009, the largest increase of any county, and the other four counties that make up New York City were all in the top percentile. By contrast, national life expectancy lengthened only 1·7 years per decade, and the USA—already trailing the world’s longest lived countries— dropped back further. “What we see in the United States sends an alarming, alarming message”, says Ali Mokdad, who led the research. “We are not catching up with what everyone else is achieving. And in many counties in the United States, we are falling behind: our life expectancy is going backward.” In this context it is all the more urgent to understand the improvements witnessed in New York City, and the lessons that can be applied elsewhere. The picture is muddied somewhat because the New York City of today is not the same city to which it is being compared in the past. Each decade, several million people migrate to the city or emigrate away, and over time this constant churning of human beings alters the character of the population. Consequently, some of the increase in life expectancy refl ects the health of the people the city attracts rather than any change it has wrought on them. “Whenever you see that people are diff erent in diff erent places, you have to ask whether it’s because people are diff erent in diff erent places, or whether the places cause them to be diff erent”, says Matthew Turner, an economist at the University of Toronto, Canada, who has researched the relationship between location and health. “It’s really hard to sort out.”