When you hear the phrase "diesel engine," you probably picture a machine that belches a bunch of gross black smoke. Diesel engines actually burn cleaner now than they did decades ago, and from a certain point of view, they're actually better for the environment than engines that burn gas. They emit less carbon dioxide (they also cost less to run).

However, they still do emit other gases, including nitrogen oxides, which are bad for your lungs. The question of just how much nitrogen dioxide they emit is important, when we're deciding whether diesel trucks are great or are choking us all. This was a question that Volkswagen said they aimed to answer in 2014, with the help of some monkeys.

They put the macaque monkeys into a chamber that was connected to Volkswagen cars running on treadmills. Though a monkey gas chamber does not sound like it makes for a pleasant image, they hypothesized that the monkeys would not suffer damages from breathing the diluted exhaust, and the monkeys were in fact in for a good time because the scientists put on a TV with cartoons for them (seriously).

The experiment turned out to be deeply unethical for one reason: the scientists didn't expose the monkeys to enough pollution. Volkswagen had installed a secret device in the engine that detected whether the car was being investigated in a lab and so switched to an alternative mode that released less NO2 than normal. This device also kicked in during various other emissions experiments, rendering a whole lot of experimental engine data unreliable (if you're been following news on diesel engines this past decade, you've probably heard about this already).

Even aside from Volkswagen's shenanigans, the experiment was not a success. The monkeys exposed to fake low-pollution exhaust exhibited lung damage anyway. And the control group, who were exposed to fresh air, also exhibited lung damage. They just were just very unhealthy monkeys all around. This might say something about the quality of air in general, or the fact that every single one of those monkeys was a loyal customer of Marlboro.

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For more monkey business, check out:

The 6 Most Baffling Science Experiments Ever Funded

Child Psychology Was Developed by Tormenting Monkeys

6 Unseen Downsides to Operating An Actual Monkey Business

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Top image: Sammy-Sander/Pixabay

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