When cities plant trees, they help reduce pollution levels and improve people’s health. Studies show that America’s trees save thousands of lives a year , mainly by preventing breathing-related problems (they also make you feel like you have more money , if you’re into that sort of thing).

Ideally we’d have trees on every street in the country because trees are good for people, but the reality, of course, is nothing like that. Cities tend to have spotty canopies, with richer residential areas generally enjoying more cover and poorer industrial areas seeing less.

You can see how trees cover the landscape of 13 cities using a nice tool developed by researchers at Portland State University. From Sacramento to Pittsburgh, it shows how urban environments are served with trees down to a micro level, and how they cover vulnerable groups like the young and the old.

“We created an online platform that says where are the dirtiest, hottest, and most vulnerable people in each of these cities. Cities can then evaluate where they might plant trees spatially at a neighborhood level,” says Vivek Shandas, one of the academics behind the maps.

The tool grew out of work by P.S.U. in Portland (which we covered last year). Based on data from 144 sensors placed around the city, it showed the effect of tree planting and the impact on people’s health, for example in reducing the rate of asthma. The tool uses public data for 12 other cities, plotting the “heat island” effect, nitrogen dioxide levels, and demographic hotspots.

Because of the way they developed, cities like Atlanta and Houston have the most opportunity for planting trees to improve health. “Houston and Atlanta sprawled quite a bit with low-density housing and connected those places with freeways. They tend to have more places for plantings than places like Albuquerque, for instance,” Shandas says.

By switching to the “plan” tab in the tool, planners can begin to understand how many trees would be needed to reach certain canopy goals, though the tool isn’t meant to be the last word. Shandas describes it as an quick reference guide so cities can get a sense of where trees are most needed.