If it seems like your seasonal allergies are getting worse over time, you're probably not wrong. Estimates are that 30 to 40% of the world's population now have some form of allergy, and medical anthropologist Theresa MacPhail says allergic reactions — including everything from hay fever to eczema and asthma — are growing in the U.S. and around the world.

MacPhail is an associate professor of science and technology studies at Stevens Institute of Technology. In her new book, Allergic: Our Irritated Bodies in a Changing World, she explores some of the theories behind the rise in allergies — including the theory that excessive emphasis on hygiene (and perhaps even showering) can contribute to the development of sensitivities.

"You've probably heard that we don't let kids eat enough dirt. They don't play in enough dirt. They're not around enough germs," she says. "We have seen that people who send their children to daycare centers, there's something about being in a daycare center that is also protective."

Other explanations for the increase in allergic reactions include the shift in our diets over the years toward more processed foods and less fiber, which affects our microbiomes. MacPhail also posits a link between allergies and a rise in exposure to environmental toxins, which could reduce the skin's ability to ward off potential allergens.

MacPhail's interest in allergies is personal: In August 1996, her father was riding in a car in rural New Hampshire when a bee flew into his open window and stung him on the neck, triggering an allergic reaction.

"Before long, my father's cells were just emitting histamine," MacPhail says. "My dad started to have trouble breathing, ... his neck started to swell up. ... Within 30 minutes he was dead on arrival at the hospital."

MacPhail says what happened to her dad is an example just how extreme the body's reaction to an allergen can be. She likens immune cells to bouncers or curators whose job it is to scan foreign objects (such as tree pollen or bee venom) and "make split-second decisions about whether or not that thing is OK."

While most allergic reactions are not deadly, MacPhail says that regardless of how mild or severe an allergy is, it inevitably impacts a person's quality of life. That might mean spending a lot of money on treatments, such as air purifiers or antihistamines of allergy-free foods, or just simply not feeling well.

"Most people with mild allergies don't sleep well, so their sleep is affected, which means they're not as productive," she says. "Their mental health suffers, like most people with a moderate allergy have some form of depression or anxiety. We can say that that's correlation and not causation, but if you're constantly lacking sleep and you're constantly not feeling your best, it takes a toll after a certain amount of time."

Interview highlights

On how, when it comes to allergens, the immune system acts like bouncers at a nightclub

T-cells are the police officers of our body, they're constantly circulating and finding things in our body that shouldn't be there. So if a T-cell comes into contact with an oak pollen, say, and it says, "I don't like the looks of this. It's gotta go." It gives that information to a class of cells called B-cells. Think of them as nightclub managers in your body, on the street that the T-cell is patrolling. And he shows a picture of this oak pollen and says, "Hey, I really don't like this guy. If you see him, let me know. Let's contact some people. We gotta get it out."

And so these B-cells ... produce cells called IgE or little proteins, Y-shaped proteins, and those are like the bouncers. But ... every IgE is unique to the perp. So at the nightclub entrance, you've got a bouncer ready to spot oak pollen, but you've got 50 bouncers out the door all looking for specific things. And so when they see it or something similar to it, they send out the signal. They alert all of the other immune cells that something's up, you got to come and take care of this guy. So that's basically going on in your body all the time.

On the hypothesis that hygiene and allergies are connected

This British researcher [David P. Strachan] did a meta-data study. So he kind of looked at all the factors involved in developing an allergy. What he found was that in families that had multiple children, the youngest children had much lower rates of allergic disease. And so he posited that that was probably because they had older siblings who got sick a lot. And so they would bring home all of these bacteria and viruses and the littlest ones would be exposed to a whole bevy of things that maybe the eldest didn't have the same exposure to. ... There was something about being the youngest that was protective.

And it's probably the same theory that you're just getting exposed to more germs on a day-to-day basis, and that, at a young age, that's actually helpful because it helps to train your immune system so it's not going to be oversensitive when the kid gets a little bit older.

On studies that show early exposure help with tolerance

By around [age] 3, your immune system is kind of set up and it's very hard to change it after that point, but it's very malleable before that point, which is why early exposures to things seems to be so protective. The landmark studies that support the hygiene hypothesis were done actually in Switzerland and Germany, where they [studied] children who were regularly exposed to dust in animal barns — and it's interesting because the animals seem to be a key component. So if you're living on a farm with livestock and you're a baby and you're being carried by your mom in and out of this barn where there are pigs and cows and ducks and dogs and whatever, you tend to have very low rates of sensitization and allergic response in those adults once they grow up. ...

It could be the allergens in the air mixed with certain types of bacteria that would be in a barn. But the animals do seem key. And I will say that if you grow up with a dog in particular, dogs seem to be protective. So people who grow up in a household with a dog also tend to have a slightly lower rate of allergies than people who grow up in a household without pets.

On exposing babies to potential food allergens

Prior to 2016, when the advice changed, we were, as just normal best practices, telling parents to not only avoid certain allergenic foods like peanuts, strawberries, eggs, milk when they were pregnant, but also to avoid giving them to their younger children until after the age of 3 — and it turns out that was exactly the wrong advice. And the way we figured that out is there is a researcher from Israel [who] actually noticed that in places that supplemented their young children's diet with ... peanut paste, they actually had incredibly lower rates of allergy to peanuts.

And so he did a more official study and did find that the early exposure seemed to be protective. But the tricky thing is it's not protective for everyone. So if you give a six-month-old baby a trace amount of peanut butter, say some of them will learn to tolerate it and some of them will still react because they might already have been sensitized through their skin. And so it's a dice throw, and the best practice now is we tell them, expose [the baby] to a tiny amount and see what happens. But we could also be seeing an earlier reaction because they've been pre-sensitized. So that's the best advice we can give for now, but it's not perfect advice.

On our diet's effect on our microbiome

Billions, trillions of bacteria live inside of our intestinal tract. Our guts are just replete with things that are not us, but that help break down foods and are the reason that we can eat food and turn them into [nutrients] and stay alive, basically. One theory about the rise of allergies is that over the last 200 years, our diets have gotten dramatically different in terms of what we eat, the types of food we eat, so more processed foods, less fresh fruit and vegetables, different foods. ... We cook differently, we manufacture differently, we grow differently ... which is a problem for the microbiota that have co-evolved with us.

So for millennia, for thousands of years, [the microbiota] got the same diet or a similar diet, and now suddenly they're being flooded with a lot more fats, a lot more sugars, a lot less fiber. ... Fiber is necessary for a lot of those good, healthy bacteria that are helping us to digest food. And so the theory goes, without all of that, there has been a difference in the composition of what types of bacteria are thriving or what ones are accidentally being starved, because they're not getting the right types of food from us anymore. And so if you change that balance in our gut microbiome, then you're throwing off the immune system itself.

On how our skin acts as a defense against allergies — and why showering less frequently can be a good thing

If you'd like to think of your skin as part of the immune system, you should, because it's basically the first line of defense. It's what keeps things out, mostly. And what we've found is if you use harsh detergents, if you put a lot of things on your skin, you are either stripping the skin ... you're killing off the good bacteria or you're disrupting the delicate balance between fungi and bacteria on your skin, causing a huge problem. Or you are just adding more things that the immune cells in your skin have to deal with.

So probably most people don't realize that there are 85,000 chemicals on the Environmental Protection Agency's Toxic Control Substances Act watch list. That's a lot of different chemicals that we've introduced into our environment that we're inhaling or we're coming into contact with through our skin or eating. ...

So we're just coming into contact with all of these things and our skin is a barrier. And so one of the theories of allergy causation is the barrier hypothesis. So if you have "leaky skin," so if your skin is more porous or is irritated, things are more easily going to get introduced into your immune system. And possibly your immune system is going to decide that thing is not great.

On house pets developing allergies

I think the fact that we are doing something that is also affecting our pets is the best evidence we have that we're really causing allergies, full stop. There is no evidence that we have that we know of, that any animal in the wild develops allergic responses. So all mammals, all animals have immune systems, but only the ones that live with us are negatively reacting the way that we are. ... So in dogs, it will be a lot of scratching, a lot of itch. In cats it can be scratching, but it can also be wheezing. A lot of cats get asthma just like we can get asthma. For birds, it's the same. It's asthma, an itch. And for horses, it's asthma and itch also.

I actually went to Cornell to their veterinary school ... and they said it's absolutely the fact that there's more allergies in pets, except that it's less diagnosed and less surveyed, so we don't have solid numbers, but they've been seeing an increase in rates and it's becoming a larger problem. Their hypothesis is that it is directly linked to lifestyle, since our pets are living exactly like we're living and we are also changing their food. So a lot of this is that we're producing their food exactly the way we produce our food.

On municipalities planting more male trees, which increases pollen

Female trees tend to be messier. So they have seeds falling and things like that, so they're harder to clean up after. And so for years it was thought, oh, well, let's just have the trees that don't have that problem except that they're pumping out pollen to pollinate the female trees. And so you accidentally got this imbalance of pollen-producing trees.

Sam Briger and Thea Chaloner produced and edited this interview for broadcast. Bridget Bentz, Molly Seavy-Nesper and Carmel Wroth adapted it for the web.