It is generally well-reported that there is a life expectancy gap between white and black Americans of about four years. But it can be hard to visualize exactly what this number means.

In a recent conversation, David Williams, a public health researcher at Harvard, described the racial gap to me in stark terms:

One of the ways to think of the racial gap in health is to think of how many black people die prematurely every year who wouldn't die if there were no racial differences in health. The answer to that from a carefully done [2001] scientific study is 96,800 black people die prematurely every year. Divide it by 365 [days], that's 265 people dying prematurely every day. Imagine a jumbo jet — with 265 passengers and crew — crashing at Reagan Washington Airport today, and the same thing happening tomorrow and every day next week and every day next month. That's what we're talking about when we say there are racial disparities in health.

One caveat: Since the study Williams referenced was done, the difference in white and black life expectancy closed — from about six years to about four years. But that's still tens of thousands of lives lost every year due to racial differences in health.

I asked Williams why there is such a tremendous gap in black and white life expectancy. He said there's no single issue to blame; it instead comes down to many factors, largely related to where people live.

"Many Americans live in places where the healthy choice is not available," Williams said. "Because if you look at a supermarket in the local poor community, they don't have access to fresh fruits and vegetables. … If it's not safe to go out and walk, levels of physical activity are not as high in a neighborhood context."

He added, "We need to think as a society, how can we make the healthy choice the easy choice? How can we ensure that every American community has access to the healthy options for good living?"

There's a good amount of research to back this up: As Ezra Klein explained for Vox, a study published in the Journal of the American Medical Association found that a "low-income resident of San Francisco lives so much longer than his or her counterpart in Detroit that it's equivalent to San Francisco literally curing cancer."

After decades and decades of systemic racial segregation, black people have been by and large pushed to areas corresponding with some of the worst life expectancies, creating huge racial gaps in health.

Again, this isn't just because of one single variable. It's a mix of issues, including how walkable a neighborhood is, how clean the air, water, and soil are, the availability of healthy foods, public health policies that push people away from bad habits or foods, and so on. Geographic location just reflects the place all those ideas come together — often in a way that affects certain groups more than others. And it shows why it's important to take a comprehensive view toward public health policy, tackling a variety of issues at once, instead of focusing solely on just one or two problems in a community.