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People can get pretty protective of sperm—which seems reasonable. After all, they are kind of important for humanity.

So you can imagine the collective shock and horror when researchers announced in 2017 that sperm counts in many Western countries were at a record low. Specifically, that analysis reported a nearly 60% decline in total sperm count from 1973 to 2011. Now, that doesn’t mean the end of our species or anything—even a 60% drop still leaves millions of sperm per milliliter.

And this wasn’t really anything new to scientists, either, who had witnessed this decline in the literature for over the last century. But it did bring up a pretty big question: What is going on? At this point, we don’t exactly know, and this drop in sperm count could be a combination of tons of factors.

But never fear: We do at least have a few researched ideas to ease your mind. The first suggestion scientists have is that some kind of environmental toxin is to blame. That might feel like a bit of a catch-all, since there are thousands of chemicals out there, but researchers are working to narrow down the options.

Some think that phthalates, which are present in a bunch of household plastic products, might be the culprit. You can be exposed to them from ingesting and inhaling these substances, or from contact with your skin, and many studies tentatively label phthalates as an endocrine disruptor. In other words, they might inhibit the body from making testosterone effectively, which in turn, lowers sperm count.

And it’s not just sperm count, either, but also overall sperm health—like how they move and physical abnormalities. But for record, more than 90% of studies that have looked at this were in mice, and phthalates have been shown to affect mice and humans differently. So maybe they aren’t as disruptive in people.

What these studies can tell us, though, is that risk of sperm damage is dose-dependent. Tiny amounts of exposure might not be a big deal, but more exposure comes with a higher risk of harm. Unfortunately, though, it’s not clear how much of the substance you’re getting during your day to day activities, like touching a shampoo bottle or drinking out of your canteen.

And besides, the type and timing of the exposure might actually matter even more than the dosage, too. Both in mice and in humans, the outcomes are different if you’re getting that phthalate exposure when you’re an adult versus an adolescent or when you’re a developing fetus. So there are a lot of variables at play here, and the relationship is still just correlational, too.

Maybe unsurprisingly, this isn’t the easiest thing to test experimentally in humans. Like we said, people can be protective of their sperm. So for now, we don’t know for sure if phthalate exposure is causing the drop in sperm count— and the same goes for exposure to pesticides and air pollution, two of the other suspects.

There can be big differences in data depending on the population, and there is always going to be variability in how individuals respond to different chemicals. Another possible factor in this dropping sperm count is obesity, but it also has some variability. For some people, it’s possible that extra fat might be warming up their gonads too much—which is a problem, since testes and the sperm within them are extremely sensitive to temperature.

Some researchers think that testes experience significantly more heat when there’s an increase in stomach and hip fat, or even from fat deposits around the scrotum itself. And all of that can lead to less healthy sperm. Plus, increased body fat influences the HPG axis, a system in the brain that’s important for hormone regulation.

And that could result in lower testosterone production, and worse sperm quality, too. But when studying obesity, researchers also need to consider the person—things like their family history or diet—not just their fat. For example, a few studies have shown that rats with metabolic diseases like obesity and prediabetes tend to have offspring with those diseases as well.

And not just because the rats keep a few too many cookies in the cage for their kids to nom on. The parent’s metabolism can actually change which genes are expressed in their offspring, making them more likely to develop metabolic issues. If this is true in humans as well, it could mean that obese parents—who potentially have fewer or less healthy sperm—could give their children a greater risk of being overweight, as well, which could lead to them having sperm problems.

Diet is also a potential factor. A high calorie diet could contribute to more oxidative stress, a process that produces free radicals—molecules that can physically damage structures in your cells. And these free radicals might be responsible for damaging the DNA within sperm.

So, again, it’s not an easy picture to put together. There’s probably a lot going on here. Now, chemical exposure and obesity both make sense for why sperm count might be dropping.

But some researchers also suggest there might be another, more surprising culprit: climate change. It’s essentially for the same reason obesity doesn’t seem to be great for sperm: they really don’t like the heat. And as the climate gets hotter, that isn’t great news for reproductive health—especially if people don’t have resources to cool themselves down.

At the end of the day, though, the drop in sperm count probably isn’t just due to one of these things. Fertility and reproduction are really complex, and it will likely take more than eliminating phthalates or cranking up the A/C to solve this international problem. Researchers will keep doing large-scale studies to figure out the broader picture, but the details will really reveal what’s going on.

It just might take a while to figure them out. In the meantime, if you’re worried about your own reproductive health, you can always talk to a doctor. And a healthy diet and exercise are never bad ideas, either.

Thanks for watching this episode of SciShow! SciShow is produced by Complexly, a group of people who believe that the more we understand, the better we get at being humans. If you’d like to learn more about things like dropping sperm count or healthcare in general, you can check out one of our other shows, Healthcare Triage, hosted by Dr.

Aaron Carroll. You can find it over at youtube.com/healthcaretriage! [OUTRO ♪].