Our world isn't just some big rock we live on.

It's an ever-changing environment, with countless creatures – us included – just trying to survive.

That's why as an engineer, you have to think about how you affect the world and how the world affects us.

If we want to try and live in harmony with nature, we need environmental engineering.

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Environmental engineering combines natural sciences like chemistry and biology with engineering principles to come up with solutions to environmental problems.

It boils down to three main things: protecting people from the environment,

protecting the environment from people, and improving the overall quality of the world around us.

As an environmental engineer, you'll be looking into issues such as public health and sustainability.

You'll be especially concerned with controlling air and water quality.

The air we breathe and the water we drink are crucial to life itself, and even simple changes can lead to serious problems.

Take, for instance, what happened in Flint, Michigan.

After switching the city's water source to the Flint River back in 2014, Flint's drinking water became dangerous.

Untreated river water corroded the pipes, contaminating the water with lead.

Lead consumption affects everything from your heart to your brain, leading to cardiovascular problems like elevated blood pressure and hypertension,

as well as neurological issues like increased aggression or difficulty thinking.

The contamination was also tied to an outbreak of Legionnaires' diseases that caused at least 12 deaths.

One study even found that fertility rates decreased by 12% and fetal death rates went up by more than half.

Solving the problem isn't just about switching Flint's drinking water back to a better source.

All of the city's damaged pipes need to be replaced before the water will truly be safe to drink again.

It will take years to fix what could've been avoided.

The lesson here is that you need to make sure whatever water source you're using, it isn't going to mess up any systems already in place.

We now know that water from the Flint River should've been treated with alkaline chemicals to reduce corrosivity before being fed into the city's pipes.

It's not just drinking water –

you need to be concerned with water quality whenever you're dealing with an aquatic environment, like a natural water supply that houses fish.

Flint's problem was rooted in human safety, but just as often it's the safety of other organisms you need to be worried about.

It's critical that your work doesn't dramatically alter the local water's natural characteristics.

Like, for instance, its dissolved oxygen content.

Fish and other aquatic creatures need to consume oxygen from the water around them in order to survive.

It's impossible to perfectly predict the amount of dissolved oxygen needed,

but in general, a level of about 4-5 parts per million is usually the bare minimum necessary to support an underwater population.

Get up to 9 parts per million and you have a thriving community, but drop below 3 and even the toughest fish won't be able to survive.

Cold water can hold more dissolved oxygen than warm water, so cooling off a lake can help to correct low levels.

Photosynthesis from the water's plants will also produce oxygen, so it might not be a bad idea to add some greenery, too.

Another thing to worry about is the pH level: how acidic or basic is it?

Pure water has a pH level of 7.

Below 7 is acidic and above 7 is basic.

Generally, a pH range of about 6 to 8 supports life.

But go too far outside of these levels and you'll start running into trouble, especially on the lower end.

Fish will usually start to die at pH levels under 5.

More acidic water can also leach metals from your pipes, such as copper, zinc, or lead – like in Flint.

Drinking water with a higher pH level isn't as dangerous,

but it can cause drinks to taste bitter and lead to mineral build-up in your plumbing, which makes it harder for water to flow.

You can try to correct out-of-range pH levels by adding other chemicals to the water, but those need to be supportive of your overall system.

For instance, if you're adding salt to try and correct out-of-range pH levels, you could throw off the water's salinity, or the amount of dissolved salt.

That definitely won't taste the best, but it reduces the dissolved oxygen; in environmental engineering, everything is connected.

You can tell how much salt is in the water by measuring its conductivity, or how well electricity flows through it.

The more salt, the more ions, and the better the water will conduct electricity.

And any hormones that make their way into a water supply can have pretty substantial effects.

Many of the medicines you take end up in your sewage and can affect entire aquatic populations.

Estrogen is especially important because it's hard to filter out.

When released at the right concentrations, scientists have seen it change the sex of entire schools of fish!

There are many other factors that affect water quality, like how clear the water is, what nutrients are in it, and if it has any viruses or bacteria swimming around.

But no matter the problem, you need to treat it.

And you don't just want to play clean-up after a disaster – you want to try to snuff out the problem at the source.

One way to remove unwanted material is through a process called coagulation.

It works by adding a chemical like aluminum sulfate that causes small particles to stick together, or coagulate.

As they clump, they become larger and easier to remove with something like a filter.

You can clean water by focusing on things like rivers and sewers.

It's a whole different problem to clean something that's all around us, like the air.

From exhaust fumes to smokestacks and insecticides, there are countless forms of airborne pollution.

Factories are among the largest culprits, producing chemicals like carbon monoxide and sulfur dioxide as byproducts of the combustion process.

Breathing in too much carbon monoxide can lead to flu-like symptoms, such as dizziness, weakness, and vomiting.

Enough exposure can even be fatal.

Every year, more than 20,000 people in the US end up in the emergency room, due to non-fire related carbon monoxide poisoning, so this is a pretty big problem.

When pollutants like these get in the air, you need to find out not just how they got there, but how far they're going to travel.

High wind speeds and storms will be able to carry the pollutants farther than a light breeze.

But probably the biggest thing you need to be concerned about is the size of the pollutants.

The smaller they are, the harder it is to filter them out.

For example, bacteria are typically bigger than viruses.

If your friend has a cold that they don't want to spread, they should wear a face mask with extremely small pores.

Remember, the common cold is a virus, so the pores need to be smaller than if they were just trying to stop the spread of everyday bacteria.

Filters can remove many pollutants from the air, but they come with some limitations.

Some of the smallest particles can slip through and larger ones clog things up, so they need to be replaced even when working properly.

That's where absorption towers come in.

They combine liquid falling from above with air rising from below to remove many kinds of particles.

Another way to remove pollutants is by using an electrostatic air filter.

These are made up of multiple layers of vented metal and essentially work like a magnet for any unwanted particles floating in the air, such as dust, mold, or pollen.

As air passes through the first layer of metal, the molecules in the air are positively charged by the friction between the air and the filter.

With their new positive charge, the molecules will attach themselves to the next couple of layers as they try to pass through the rest of the filter, essentially getting trapped inside.

So not only will a good air filtration system help block out deadly pollutants, but it can also help save you from a runny nose!

Air and water quality are just two of the problems addressed by environmental engineering,

but others include things like noise pollution and waste management.

Loud noises may not sound that important, but spend some time trying to think next to a construction site, and you'll realize how harmful noise pollution can be.

And many of the things that we make, make noise.

Cars are a big one.

Left unattended, highways can be a major source of noise pollution.

That's why you'll see those wall-like sound barriers on the side of the road.

Factories and places with a lot of loud machinery can also disrupt their neighbors.

Earplugs may be enough for the workers inside, but if a factory is making too much noise,

it might need a more sound-proof building or to be put in a less residential area.

You also have to find a way manage all of the trash you create, whether that's piling it up in landfills or finding ways to recycle.

Recycling isn't just about reducing pollution – it also helps conserve natural resources like timber and saves energy, which might come from fossil fuels.

It's not only small things like soda cans, either.

Computers, cars, and even abandoned buildings can all be recycled.

Many of the chemical reactions involved with engineering also produce waste, so you need to get rid of that too, especially if it's hazardous.

Sometimes you can treat the hazardous materials and eliminate the danger outright,

but other times you might have to settle for simply storing it as best as possible.

If you have to do that, you need to make sure that the waste won't react with, weaken, or dissolve whatever container you put it in.

All of these things – and so much more – must be considered when approaching problems from an environmental engineering perspective.

But as long as you take the proper precautions, you'll be on the right path to keeping this world spinning.

Today we learned all about environmental engineering and the work that goes into managing our impact on the environment, and its impact on us.

We found out the importance of water quality and how differing levels of dissolved oxygen, pH, and salinity can affect it.

Then we saw how air quality matters and the different types of pollution we have to deal with.

Finally, we went into other issues, like noise pollution and waste management, and saw how they can affect us too.

I'll see you next time, when we'll learn about alternative energy.

Thank you to CuriosityStream for supporting PBS Digital Studios.

CuriosityStream is a subscription streaming service that offers documentaries and non-fiction titles from a variety of filmmakers, including CuriosityStream originals.

For instance, CuriosityStream has “Nature's Mathematics,” a series about patterns and symmetry in nature.

Nature's Mathematics explores the rules that shape trees and river estuaries alike,

and that continue to baffle scientists by their often unfathomable ubiquity.

You can learn more at curiositystream.com/crashcourse and use the code crashcourse during the sign-up process.

Crash Course Engineering is produced in association with PBS Digital Studios, which also produces

Hot Mess, a show about how climate change impacts all of us, and what we can do about it.

Head to the link in the description to find out how we can create a better future for our planet and ourselves.

Crash Course is a Complexly production and this episode was filmed in the Doctor Cheryl C. Kinney Studio with the help of these wonderful people.

And our amazing graphics team is Thought Cafe.