- This tree releases a carcinogenic gas that reacts with car exhaust to form another gas that could send me to an early grave.

So time to die tree.

(chain saw buzzing) On March 21st we get this email from a subscriber named to Andrew and actually come to think of it, we never replied and so consider this video our reply.

Anyway, Andrew works for a city engineering department and he was saying that he was getting into fights with his coworkers about trees.

Specifically one of his coworkers said that trees can actually make air quality worse.

And so Andrew Googles this information and he discovers that quote, "Trees release a chemical called is isoprene "that can worsen air quality.

"This goes against everything I thought I knew about trees."

And I gotta say, me too, Andrew, me too.

And then a few months later a paper on exactly this topic comes out in an ACS journal called "Environmental Science and Technology".

Now I have to say I didn't know much about isoprene.

So I decided to dig into it.

This is isoprene, it's just carbon and hydrogen and this simple little molecule is the problem with trees, according to Andrew's coworker.

So first question, do trees actually emit isoprene?

The answer here is yes, they do.

And it's not just trees.

Look at this biology figure.

This is a list of all plants that produce seeds and each word in this list represents an entire order of plants.

The black lines are orders that have species within them that release isoprene, the red ones don't.

So eyeballing this, I would say roughly about, I don't know, a third of all plants that produce seeds emit isoprene globally.

Plants release this much isoprene into our atmosphere every single year, which makes is supreme the second most emitted volatile organic compound after methane.

Now that is a wild fact, because is supreme is very important to living things like look at all these molecules.

This stabilizes cell membranes.

That deters predators.

This is vitamin A.

These are the two primary human sex hormones.

This could help you outrun a lion and that is rubber.

And all of these molecules and lots more are made from isoprene, which makes isoprene basically a giant molecular Lego kit.

Now, isoprene is not as famous as DNA or RNA, but it is not a stretch to say that it is one of the fundamental building blocks of life.

So why are plants just spewing it into the atmosphere as if it has no value whatsoever?

We don't really know why trees let this valuable molecule just evaporate off into the atmosphere.

But we do know that trees release exponentially more isoprene the hotter it is.

So one theory is that they do that to deal with the heat.

So point for Andrew's colleague, trees do release isoprene.

Now the question becomes is that bad?

This is 1.7 grams of salt, just plain old table salt.

And this amount is the LD 50 for rats, meaning it will kill 50% of lab rats that administered to.

This is 1.2 grams of isoprene.

It's actually water, but it's what 1.2 grams of isoprene would look like if it were in this syringe.

And this will also kill 50% of lab rats it's administered to.

So isoprene is only slightly more toxic than table salt.

Now if we assume that isoprene is about as toxic to humans as it is to rats, and if we do some math, we can actually calculate that you are more likely to be struck by lightning and eaten by a shark at the same time than you are to be exposed to a lethal dose of isoprene from trees.

That is just what the math says.

All right?

Which should be obvious, because you could live your entire life in the densest forest and still never be killed by isoprene released from trees.

You could live here and you'd be fine.

But if you look at page 1015 of monograph 71 published by the International Agency for Research on Cancer, you will find that is supreme is definitely carcinogenic to rats and mice.

We don't know for sure about humans yet 'cause there's not enough human data.

So IARC classifies it as possibly carcinogenic or group 2B and we will come back to that 'cause it is important.

But for now, let's look at another way that isoprene could be bad for air quality.

Isoprene has an organic molecule and it is released into a very oxygen-rich atmosphere.

So it gets oxidized.

Now the final oxidation product is carbon dioxide because that is the most oxidized that a carbon atom can possibly be.

But in between is supreme and CO2.

There are hundreds of steps, maybe even thousands of steps.

And when I say that I'm not exaggerating, look at these reaction mechanisms.

There are an outrageous number of steps here.

Anyway, lots of these different reactions are activated on hot sunny days, because light from the sun splits up atmospheric gases to form chemical species called radicals.

Now radicals have an unpaired electron, which means they are very reactive, which means they'll react with just about anything including isoprene and other volatile organic compounds.

Now that reaction forms these degradation products.

There are a lot of them.

That's why I've just put the words on the whiteboard.

Those though can react with nitrogen oxides like NO and NO2 to form ozone.

Ozone way up in our atmosphere, that's great.

Up there it helps protect us against UVB rays.

But ozone down here is bad for anything that breathes oxygen and nitrogen oxides come from burning stuff in air like this car engine does.

Some of the nitrogen gets oxidized so you get NO and NO2.

Now cars are actually not the worst emitters of NO and NO2.

They used to be really bad.

But then the EPA started requiring catalytic converters.

Some of the worst emitters of No and NO2 are auxiliary power plants.

When a city demands more power than its regular power plants can supply, the auxiliary power plant kicks on to provide that extra power, which it does by burning diesel, which releases lots of NO and NO2.

When it's hot outside, everyone turns on their AC, which burns more fossil fuel, which releases more nitrogen oxides, which react with the exponentially higher levels of isoprene being released by all the trees 'cause it's hotter.

And because of all that extra heat the reaction goes because of the Arrhenius equation.

So everything that we've just talked about, all of that means that on hot and sunny days in cities, there is way more ground level ozone and that's not good for lungs.

This thing that I'm holding, by the way, this is an ozone test strip and it is saying that the current ozone level is less than 90 micrograms per meter cubed, which is safe.

You can check ozone levels at airnow.gov and that is free, unlike this.

Trees release isoprene, which can react with nitrogen oxides from fossil fuels to form ground level ozone, which can in fact make air quality significantly worse on hot days.

So no more trees I guess.

When I was in high school, I did a summer internship at the National Institutes of Health.

And I'll never forget one of the main things my mentor taught me, which is this.

In science you find what you are looking for.

For example, if you go looking for injuries caused by seat belts, you'll find them.

But that doesn't tell you whether seat belts make you more or less safe.

If you are looking for potential carcinogens in a loaf of bread, you'll probably find them, but that doesn't tell you whether or not the bread is healthy.

If you are looking for ways that your partner annoys you, you'll probably find those too.

- [Partner] I annoy you?

- But that doesn't tell you whether or not you're in a healthy relationship, which I am.

Same deal with trees.

We started this video by looking for ways in which trees are bad.

So it's really not surprising that we found them, but we found them by asking two very narrow questions.

Instead of asking what we really care about, which is, are trees in cities good or bad for humans?

Now on the con side, we've got the isoprene and ozone thing, which we already talked about.

We've got the cancer thing, which we are yet to talk about.

It is important and it is interesting, and we've got other stuff like roots can damage sidewalks, roads, underground water, pipes, trees can fall on stuff, people, cars, buildings.

And last week our producer Andrew, had an acorn fall on his head and he wants you to know that it hurt.

(acorn banging) Let's add some pros to the list.

Now we're not gonna talk about all of these, but let's pick one.

For example, the top one.

Cities are much hotter than the countryside because all this concrete asphalt absorbs sunlight and then reradiate that energy as heat.

Tree leaves absorb sunlight and use that energy to make sucrose so the photons don't get converted into heat.

Trees also use the energy to pull relatively cool water out of the ground, and then they use that water as a heat sink, pushing energy into it and causing the water to evaporate.

A single oak tree can evaporate 150,000 liters of water every year.

All that evaporation cools the area around the tree down just like sweating cools you down.

This is Colonial Village.

It is DC's coolest neighborhood.

About two thirds of the land area is covered by tree canopy.

Average temperatures here are 4.4 degrees Celsius lower than the city average, and a full 9.1 degrees lower than here in Parkview, DC's hottest neighborhood where the tree cover is only around 22%.

Now look, I know we don't normally do Fahrenheit on this channel, but that means that if it were 105 here, it could be 89 here.

That's the difference between annoyingly hot and dangerously hot.

And you will not be surprised to learn that houses here cost more than double what they cost in Parkview.

Now let's come back to the cancer thing.

The lowest dose of isoprene that reliably caused cancer in mice was 140 parts per million.

Now, if you were in the middle of an oak forest, which is the place in nature where you would be exposed to the most isoprene from trees, you would be breathing in 16 parts per billion.

So we have to add three more zeros down here.

Now to get from this concentration to this concentration, you have to multiply by 8,750.

That is a lot.

And if I want it to be even more anal retentive, which let's be honest, when do I not?

I would point out that the concentration of isoprene that you right now are exhaling as you watch this, the concentration of isoprene that is coming out of your mouth is 100 parts per billion or higher than what you would experience if you were in an oak forest.

So the probability of getting cancer from isoprene released by trees, it is zero.

We can cross that off our list.

Let's come back to the main con.

Isoprene, nitrogen oxides, ground level ozone.

The answer here is not to cut down city trees, it's to reduce or eliminate our nitrogen oxide emissions, which we can do by adding catalytic converters to stuff or electrifying everything.

Then we can enjoy the main benefits of trees and get rid of the ground level ozone problem.

And by the way, in the meantime, those benefits far outweigh the risks of ground level ozone, like it's not even close.

So Andrew, our view here at Reactions is that your coworker was correct about the minor point while also being completely wrong about the major and more important point.

Sorry it took us so long to reply.

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