Is density always better for the environment?

Denser cities are better for the climate. That's not debatable. Sprawl harms the climate by consuming more natural habitat and inducing more driving.

A new study published in Lancet, the health journal, reaffirms the climate benefits of density, but also finds some counterintuitive harms. From City Lab:

Europeans living in the densest cities suffer from poorer air quality, higher mortality rates and worse heat island effects than residents of sparser urban areas on the continent.

...The authors divided more than 900 European cities into four groups distinguished by population density and available green space, finding that a person living in one of the more sparsely populated and greener cities — such as Uppsala, Sweden — produces nearly 50% more carbon dioxide than a counterpart from one of the densest places like Paris. Single-family homes use the most energy per capita out of any type of dwelling, and areas that are more spread out often have a greater dependency on cars, both of which heighten per-capita emissions.

But when it comes to health, these sustainability benefits were undercut by high concentrations of cars and other pollutants: The most compact cities saw more than twice as much traffic and significantly higher concentrations of nitrogen dioxide — an air pollutant released by burning fuel — than low-density cities. Places with little green space saw an accentuated surface heat island effect — the difference in the temperature of the ground inside and outside the city.

So, what's interesting is that cities that are best-equipped for transit and walkability also tend to suffer the most from cars. So the people living car-free existences in London are likely inhaling more tailpipe emissions than those living in car-dependent suburbs.