City dwellers are thought to be, on average, healthier than their rural counterparts. This so-called urban health advantage is usually attributed to better access to health care and improved overall infrastructure, such as clean water, safety and education.

But many of the globe's cities are already bursting and actually offer a far worse quality of life to those on the lower end of the income spectrum, whose plights can get overshadowed in large-scale statistics. Can the urban advantage be extended to more of the world's population as cities continue to grow in the coming decades?

"While cities have the potential to be healthier places for their citizens, this requires active planning," Yvonne Rydin, of the University College London's Bartlett School of Planning, said in a prepared statement. In a new report, published online May 29, she and her co-authors on the University College London/Lancet Commission on Healthy Cities examines several initiatives across the globe—from providing gardening plots to urban residents in Cuba, to improving the pedestrian experience in Bogota, Colombia—to see what's working and how we can best prepare for continued urban growth.

On the plus side, the density of cities can encourage walking or cycling over driving, thereby increasing physical activity of its residents and decreasing air pollution. On the minus, it can also lead to harmful outcomes. For instance, the heat island effect can increase ambient temperatures and put more people at risk during heat waves, and decreased access to arable land can make a healthful, produce-rich diet expensive.

Good planning can heighten the advantages of cities and mitigate their downsides. "There should be an emphasis on experimenting with and learning from diverse urban health projects," said Rydin. In Bogota, closing major streets to car traffic on Sundays and holidays has to led a huge increase in physical activity—with more than 40 percent of adults getting at least 3 hours of exercise at a time—that researchers estimate will pay back in long-term health care savings (some $3.23 to $4.26 per each dollar spent on the program). And in Cuba, individual and collective urban farms have increased access to healthful, naturally grown food for city residents for decades, with some 4 million tons of vegetables being grown inside Havana's city limits each year.

The authors conclude that urbanization alone will not automatically help everyone lead healthier, happier lives. Already about 1 billion people live in urban slum-like conditions, and that number could double in the next 18 years. The tide of urbanization is not going to raise all proverbial ships. "Economic growth cannot be assumed to lift all urban citizens into a zone of better health," Rydin said. "In many urban areas, rich people and poor people live in different epidemiological worlds, and the burden of ill-health is highest in the poorest groups. The double burden of communicable and non-communicable disease is borne predominantly by poor people." To improve the health of all urban dwellers—present and future—concerted planning will be necessary, but global payoffs will be great.