I live close to a large, gorgeous city park. I go on long walks there almost every day if it is not too rainy or too sweltering or the air is not too clogged with pollution. Some days, I have no symptoms at all and I enjoy the park with impunity. Other days are almost unbearable.
When I return home, my eyes sting and itch and if I dare try to touch or gently rub them, I trigger a sneezing fit that may last as long as 30 minutes. Sometimes the surface of my eyeballs burns so much that my eyelids reflexively pinch shut, my conjunctivitis producing so many involuntary tears that it looks as if I’m in the middle of a nasty emotional cry.
On the bad days, I open a weather app on my phone and check pollen counts, doing some quasi-scientific sleuthing to see what might be causing my intermittent misery. It always says the same thing: very high levels of grass. I assume I must be allergic to one of the species of grass in my area, but who knows which one.
Allergy symptoms have always been a sign of something gone amiss. Our collective allergy symptoms – all our runny noses, scratchy eyes, raw skin, upset stomachs, unhappy intestines, swollen esophagi, irritated lungs, and difficulty breathing – are trying to tell us something important about the overall health of our immune systems in the 21st century, about how we live our lives, about how overwhelmed our cells often are by our environment.
Looking at the data that we have from the last century, there’s a consensus that hayfever rates in the US increased in the mid-20th century. Data suggests that the incidence of asthma increased beginning in the 60s and peaked sometime in the 90s. Since then, rates of asthma have remained fairly constant. For respiratory allergic diseases and skin allergy, the levels likely increased over the past few decades. For food allergies, the rise in global incidence rates has been the most dramatic and visible, beginning in earnest in the 90s and growing steadily ever since.
Dr Scott Sicherer, the director of the Elliot and Roslyn Jaffe Food Allergy Institute at Mount Sinai Hospital in New York, has seen the rise of food allergy up close.
When he began work at the institute in 1997, his team conducted a study that showed one in 250 children were reporting a food allergy to peanuts or tree nuts. By 2008, that rate had more than tripled to one in 70.
“I didn’t believe the 2008 study at first,” he told me. Sicherer initially thought the rate reflected a problem with the study’s methodology – that is, until he saw similar numbers coming from Canada, Australia, and the UK, all showing that about one per cent or more of children had a peanut allergy.
Today, Sicherer has no doubt that allergies have increased over the past few decades. “We’re also seeing less food allergies being outgrown and more emerging. The severity might be intrinsically no different than it was 20 years ago, but with more people affected, it’s a big deal,” he says.
While all of this data is compelling, perhaps the most convincing evidence we have for the rise over the past 30 years is hospital admissions. Every two hours, someone with a severe allergy ends up in the emergency room. Those numbers seem like incontrovertible evidence that the problem of allergic diseases is expanding. According to researchers at Imperial College London, food anaphylaxis hospital admissions increased 5.7 percent (from 1998 to 2018), while fatalities decreased from 0.70 to 0.19 percent. During the same period, prescriptions for adrenaline auto-injectors, or EpiPens, increased by 336 percent.
Researchers can all agree on one thing: allergies have got worse over the past few decades, and the numbers of allergy sufferers worldwide is likely to keep growing apace. We also know that people are experiencing worse symptoms and longer allergy seasons. We are in the midst of a growing global epidemic of allergic disease. Why?
It is not just the copious amounts of microscopic tree and grass pollen that are circulating in the air right now, floating around on the breeze, lightly coating outdoor tables or caked onto the exterior of cars and trucks like yellow dirt, but also the combination of dust, ozone, nitrogen dioxide, sulfur dioxide, and other particulate matter too small to see, even with a microscope.
All the detritus of our modern civilisation is constantly swirling around our bodies, being inhaled along with pollen deep into our lungs — and that pollution is especially concentrated in the urban air surrounding our cities. Even on beautiful winter days, when no pollen and mould spores are circulating, the air is still replete with things that are irritating to our immune systems. Is all the pollution in the air we breathe making our allergies and asthma worse? Could environmental changes – to both our natural landscapes and the climate itself – over the past 200 years be driving the dramatic recent increases in allergy rates around the world?
In short, the answer is a resounding yes. But frustratingly, I need to include “sort of” as a coda. Like with our genetics, changes to our natural environment – or the physical landscapes in which we reside – appear to be at least partially responsible not only for an increase in allergy rates, but also for a worsening of our normal seasonal allergy symptoms.
Physician researchers working on hay fever and asthma in the 19th century suspected that shifts in agricultural production and polluted urban environments were directly linked to the development of their patients’ hypersensitivity, or allergy. These early scientific theories about environmental allergy causation eventually paved the way for what would become known, more than a century later, as“the hygiene hypothesis.”
The hygiene hypothesis posits that changes to our environments – specifically a lack of exposure to a wide variety of microorganisms early in our childhood development – can lead to an overactive immune system. The basic assumption is that the natural environment matters deeply to the development of our allergic irritation. What our bodies are regularly exposed to – or not exposed to – has a significant and lasting effect on our immune functions.
Several climate factors are currently intersecting to compound the problem. Most obviously, the temperatures are warming. Spring seasons are, on average, happening much sooner – beginning as early as February in some places – so plants and trees that respond to warmer temperatures are flowering earlier, too. At the other end of the growing season, fall temperatures are much milder, which allows plants to flower for extended amounts of time.
Pollen levels, in general, are expected to double by 2040, and the pollen will be more “potent” (its peptide levels will rise, likely worsening our immune system reactions).
In addition, research done at the Mayo Clinic linked climate change to an increase in CO2 levels that led to an increase in fungal growth. The study found that exposure to fungus lowers cell barriers, causing cell inflammation that can worsen allergies.
Climate change is also causing worse flooding and higher temperatures around the world. And that means more mould, as we’ve already begun to witness in places like New Orleans, where allergy rates began to skyrocket after Hurricane Katrina.
Climate change is also altering weather patterns, and storms exacerbate respiratory allergy and asthma symptoms – a phenomenon called thunderstorm asthma. Rainfall ruptures bioaerosols and lightning strikes fragment pollen grains, then increased winds distribute those ruptured fragments for miles and miles. In 2016, a thunderstorm asthma event in Melbourne, Australia, sent more than 10,000 people to the emergency room with difficulty breathing over two days.
All of this gives credence to the argument that changes to our natural environment have been, are, and will continue to affect our immune system function, causing worsening allergies. But then what caused the equally dramatic rise of eczema, or skin allergies, and food allergies? Is our natural environment to blame for them as well?
Dr Elia Tait Wojno, immunologist at the University of Washington, said, “It’s complicated.” Tait Wojno studies allergies in dogs and points to the fact that our pets and farm animals also experience allergies to support the idea that it’s our total environment causing the bulk of the problem. It’s not just our immune systems that have gone wonky but theirs, too. “I think there is an argument there that there is something going on environmentally,” Tait Wojno said, “whether that’s food, industrialisation, chemicals, toxin, just all of the above, in an evil combination.”
While the environment we live in – the natural and man-made world around us – is most definitely contributing to the problem, it isn’t the sole cause either. What is happening to us is a result of all the things we’ve been doing differently over the past 200 years and their effects on the environment and on our own biology. It’s both that simple and that complicated.
When I interview allergy experts, I force them to come down on a side. Many say the hygiene hypothesis is correct – and it remains one of the front-runner theories. But others think it’s our diet, that changes in the way we grow and prepare food have altered our gut microbiome, fueling allergies. Still others argue that the various man-made chemicals and plastics we come into contact with on a daily basis are making our immune systems more irritated. What everyone can agree on is that our gene-environment interactions (also called epigenetics) have a large role to play in the rise of allergies, as does the make-up of our nose, gut, and skin microbiomes.
The ways in which we produce, prepare, and eat food; the modern work culture with its continuous lack of sleep and high levels of stress; the antimicrobial agents, antiparasitics, and antibiotics that we use in human medicine and feed animals; gardening and our obsession with having a lush, grassy yard – all of these are suspects in the development and steady rise of allergies.
While blame for allergy may have shifted from neurotic behaviors and anxious personalities in the 19th century to our diet and microbiome in the twenty-first, our culture and daily habits have come under steady scrutiny for the roles they may play in our increasing irritation for more than 200 years. When all is said and done, we are right to blame ourselves, at least partially. Our entire modern lifestyle is likely at the root of the recent rise in allergies.
Eczema expert Peter Lio of Chicago would like people to stop looking for a root cause. He argues that it’s the wrong question to be asking. But often, his patients don’t want to hear the truth – that the cause is much more complicated than any one theory might suggest. “I tell them it’s a big mess,” he said. “There’s a skin barrier thing, an immune system thing, and there’s something about the nerve endings, and then there is a behavioral piece. .”
The food allergist Pamela Guerrerio also wanted me to underline that, ultimately, it is wrong for researchers to even look for a single cause. It sends an inaccurate message, she argued, that we just need to figure out what the problem is and then we will solve all of this. “My message would be that there isn’t one cause,” Guerrerio said, “and that we need to understand that there is probably a genetic susceptibility, and on top of that is environmental exposures, and what’s true in one group to account for their increase in allergy may be different for another group.”
At Cincinnati Children’s Hospital, Dr Neeru Khurana Hershey summed it up best: “There’s no one thing. If there were one thing, we would’ve found it and figured it out. It’s a combination of things and it’s different for different geographic areas and different genetic backgrounds. Sometimes it’s easier to put the blame on something than it is to take a hard look at what we’re doing and how we’re contributing to the problem. Because everyone’s contributing to the problem. What are we doing as a society that we should be doing better? And what am I as an individual doing that contributes to that? These are not easy questions or answers. These are hard ones.”
Theresa MacPhail is a medical anthropologist and writer. Her book Allergic: Our Irritated Bodies In a Changing World (RandomHouse) tells the story of the global rise in allergies over the last 200 years. Extract published by Allen Lane at £25.Copyright © Theresa MacPhail, 2023