Streets Without Trees are Dangerous

With more cities developing Vision Zero initiatives to eliminate road traffic fatalities, more people are seeing the way streets that are designed as a life-and-death issue. That entails seeing things that used to look fairly benign – a 13′ lane width, for example – as hazardous. We should start seeing the absence of mature street trees in a similar light.

Specifically, we should understand that urban streets without trees are very bad for us. Not “get hit by a truck” bad, but “slowly and certainly damaging your heart and lungs” bad, so – you know – not great! And the good news is that it’s not hopeless – recent research that I share below makes the case that growing trees in the specific places where we spend most of our time in the city makes a big, measurable difference.

Why aren’t we doing this already? I think we’ve accepted a lack of mature street trees as a landscaping bummer, and someone else’s problem. It would be nice to have beautiful trees, but what are you going to do, and oh, look at the time. Collectively, we’re people who are getting more and more conscious about what we eat and drink, but take unnecessary risks with the 11,000 liters of air each of us breathe every day.

I had a fun coincidence this week between a book I’m reading on another topic, and something I stumbled across on a walk in central London that reinforced that this challenge can be seen as part of a repeating pattern. As human beings, we have a long history of being more adept at addressing hazards we can see. We (generally) can’t see air pollution anymore, it seems like a big, unsolvable problem, and clear air looks safe, anyhow. But we can learn to be appropriately skeptical of clear air and motivated to improve it in specific places because we accomplished the same thing with clear water – not all that long ago, and not far from where I am right now.

Soho, London, 1854

“There were practically as many theories about cholera as there were cases of the disease. But in 1848, the dispute was largely divided between two camps: the contagionists and the miasmists. Either cholera was some kind of agent that passed from person to person, like the flu, or it somehow lingered in the “miasma” of unsanitary spaces. … most physicians and scientists believed that cholera was disease spread via poisoned atmosphere, not personal contact. One theory of published statements from U.S. physicians during the period found that less than five percent believed the disease was primarily contagious.”

The Ghost Map, Steven Johnson, p. 69

164 years ago, London was the largest, densest city in history, and people were dying because they didn’t understand that clear water from their public wells wasn’t safe.

Of the two basic camps described above, the miasmists come off worse in retrospect. They were proven wrong, for starters. And their theories about “bad air” springing up here and there without a logical connection to contributing factors was more mystical than scientific. But neither side really knew what they were talking about, and it is amusing that there were experts on cholera before anyone knew the first thing about it.

In another sense, it wasn’t their fault – they couldn’t see the bacteria. They couldn’t understand the nature of the proble. Until a guy named John Snow figured out the cholera outbreak that killed 500 people in 11 days was being transmitted through drinking water from a particular well in Soho (despite his contemporaries’ claims that he knew nothing).



Tellingly, even the guy who figured it out was expecting to be able to see the problem:

“Snow’s expectation was that contaminated water would have a cloudiness to it that was visible to the naked eye. But his initial glance at the Broad Street water surprised him; it was almost entirely clear. He drew samples from the other pumps in the area. Warwick Street, Brandle Street, and Little Marlborough Street. All were murkier than the Broad Street water.” The Ghost Map, Steven Johnson, p. 76

The problem was urgent, specific, and invisible. And – thanks to Snow’s work and the development of the field of epidemiology that was spurred by it – our society broadly believe in bacteria and other invisible water-borne hazards. And we changed our infrastructure to make it less dangerous.

As a result, if you sit me down in a lab, hand me a beaker of clear liquid, and ask me if I’ll drink it without knowing where it came from, I wouldn’t do it. I expect few people would, because we’ve collectively learned that we can’t trust our eyes with the water we drink.

What about the air we breathe?

Soho, London, 2018

On to the less dramatic part of the coincidence. In the midst of the hottest summer in London for 42 years, I had a pressing problem of my own this week. I needed a hat.

It’s a glamourous life. Tired of squinting in the hot English sun, and annoyed at the dominance of Yankees caps in the market over here, I decided to channel my homer instincts and get a Twins cap. So – totally unrelated to research for this piece or sightseeing – my son and I took the tube 20 minutes into the center of town on Monday morning.

As we were walking under clear blue skies down Air Street in Soho, I noticed something I had been planning to make a special trip to see.


A CityTree. Using moss, solar panels, and sensors, the manufacturer claims that the CityTree has the air cleaning power of 275 trees. The common street tree has been disrupted.

This is the first one in London, and it debuted to a lot of media attention in March of this year. Roughly 20 have been deployed worldwide. It looks cool. But why is it here, specifically? Why did its sponsors pony up £20,000 for it, and site it in Soho? Did a decisionmaker like the Air Street pun as much I do? Is it a coincidence that they put the big air purifier outside the Whole Foods, magnet of h/wealthy consumers that it is?

I don’t know the details of the purchasing and siting decisions, but the manufacturer claims to use a sophisticated data-led process to maximize air quality benefits. I imagine that’s true. But it’s also the positive formulation. The other way to put it is that despite this neighborhood turning from a slum to a tony retail district, and 160 years of scientific progress, the CityTree is here in Soho – a five minute walk from the old Broad Street well – because this place is still making people unhealthy. The miasma is real after all.

The Air You Breathe

The CityTree is in Soho because the air quality in the area is bad. Just look at the red knot in the middle of this scary map.



Red means premature deaths, higher incidence of respiratory diseases, all of it. If you live in a different city, I expect there’s a scary map for your city, too. Structurally, I should go into detail about why bad air is bad, and why it’s a big global problem, but I can’t do that to you.

In its typical formulation, global air quality is a big, chronic, enervating issue that’s not a top-level concern for many people. If you have the mental and emotional capacity to take on “the world’s single largest environment and human health threat” overall, more power to you. But I promised you at the beginning that all you need to focus on in reading this piece is being selfish about your health, and here’s why.

The good and bad news is that – in addition to being global – it turns out that air quality is extremely local. That makes it an issue for which local action can have personal benefits for you.


The findings from a research study about air and street trees that was published late last year got relatively little attention (that I could see), but it illustrated this dynamic vividly for me. The researchers looked at the cardiovascular systems of people who exercised on streets in London with mature trees versus those who exercised on streets without them. The people who exercised among trees in London, with its famously bad air, experienced cardiovascular benefits. The people who exercised on streets without trees didn’t. It was that clear cut.

I’ve been working on street trees and related topics for years, and I honestly had no idea the air quality benefits of individual trees were that specific. I’m used to looking at air quality maps like the one above and feeling vaguely ineffectual making small greening improvements, but it turns out they aren’t futile at all. Based on these findings, if we grow trees on the specific streets and urban places where we spend time, the air we breathe will be cleaner and we’ll be healthier.

This dovetails nicely with other research published last year out of New York that found “[t]he true impact of air pollution has been obscured by the failure to consider people’s exposure as they move around during the day… for work or recreation.” My understanding of these findings is that we’ve been quantifying and addressing air quality issues based on assuming that people stay home. This has resulted in an under-appreciation and under-prioritization of air quality improvements in places where people spend their time during the day (i.e. Soho, downtown Mpls). That means that busy urban places are extremely high leverage places for improving public health through greening.

Taking action

What can you do to improve the quality of the air you breathe in the public places where you spend time? Simple: just start noticing street trees more. Then start asking questions about how street trees are being planted and maintained in those specific places. Get as detailed as you want, but just start by asking the stewards of your urban forest (i.e. elected leaders, arborists, right-of-way managers) what’s being done and what more can be done.

This requires stepping over one cultural hurdle: it can feel a bit odd to advocate for trees in front of private property that isn’t yours. We’re used to minding our own business and not meddling with other people’s landscaping. And there is always a relationship of some kind between a private property owner and the landscaping in the adjacent right-of-way, especially when it’s in a yard or boulevard that private party actively maintains. But it’s very important to start seeing those street trees as public property and a public resource that directly impacts your health, because it’s true.

Cross-posted at

About Ben Shardlow

Urban planner/designer from MN, temporarily in London.