The first heat wave of summer hits like a wall. One day the air is crisp and cool, and the next it’s impossible to go out without sweating through a shirt. The store feels like it’s on the far side of the Amazon, or maybe the Gobi. And yet, the thermometer reveals a number that is not all that high, compared to later in the season. So why is the heat so unbearable? 

Scientists who study temperature regulation have found that the body takes time to adjust to seasonal changes and other thermal changes: a process called heat acclimatization. Research suggests that in people who regularly spend time outside, the body adjusts how it sheds heat to make the environment less stressful. “We can adapt to the heat we're in, and it's obviously progressive,” says Julien Périard, a professor at University of Canberra in Australia who studies the phenomenon. Your body in the summer, it turns out, might be fundamentally different from your body in the winter.

How does the body adapt to seasonal temperature changes?

Spending most of your time indoors in the air conditioning might slow the body’s changes, come summer. “But if we spend time outside exercising, or even just being physically active outdoors, we'll adapt to those conditions,” Périard says.

When you go for a walk on a hot summer day, before you’ve adapted to the heat, both your skin temperature and your core temperature may go up. That sets off alarms in the body, announcing heat stress. “With that, we trigger lots of sweating, and we increase our skin blood flow,” says Périard. Sweat evaporates from the skin, cooling skin down, and blood sent to the surface of the body helps shed heat. 

The volume of blood pumping through the body also goes up. More blood volume allows more heat to be shed and supports greater sweating without dehydration. There are also changes at the level of the cell, with some proteins’ production going up to protect normal functioning in greater heat. The process of reaching a fully adapted state might take a few weeks, although the precise details will depend on the situation. 

These changes can be triggered in advance, with a little bit of exposure at a time. In fact, athletes and people working in warm conditions, like utility workers out in the Texas sun all day, may slowly ramp up their time in the heat to stay healthy under strain. 

Does adapting to heat reduce the risk of heat stroke?

Being adapted to the heat lowers the risks. But it isn’t carte blanche to go for a run at noon in Havana. People who’ve adapted are still vulnerable to heat-related illness, like heat stroke and heat exhaustion, says Glen Kenny, a professor of physiology at the University of Ottawa in Canada. 

What’s more, researchers who study heat adaptation are worried about the stresses that the future climate will bring, when warming may be so swift, and so extreme, that the body can’t react fast enough. In an experiment meant to mimic an extreme 3-day heat wave in 2021 that killed 619 people in British Columbia alone, Kenny spent three days at 104°F in a special chamber in his lab, which allowed him and his colleagues to monitor what happens to the body. “Over three days, you'll see that the body will show some progressive adaptation. But in this situation, you're seeing a decay in the physiological system supporting the maintenance of core temperature,” he says. He lost about 10 pounds in three days because he couldn’t hydrate fast enough. “You're losing so much sweat. You're seeing cell function deteriorate.” 

What happens if you lose your heat adaptation?

Spending a few weeks in a cooler climate can mean the body will roll back some of these changes. “But what's interesting is we have some sort of heat acclimation memory,” says Périard. “If within a month, for example, I've removed you from the heat, and then I've put you back in the heat, in three or four days you'll probably get adaptations that are similar” to what would have taken ten days to build up originally.

Does your body adapt to the cold, too? Yes, but there’s a lot less to work with, says Périard. “We've evolved to be tropical animals, so our heat-loss mechanisms are very potent. But if you think of conserving heat, the only two things we can do are physical constriction—to try and keep the warm blood in the central circulation, essentially—or shivering, which produces a bit of heat but not too much.” Exercising can warm you up, but an essential feature of human culture—clothing—has a far more important role to play here than biology. 

“You can exercise all you want. But if it's -40° outside, you're going to get cold pretty quickly if you don't have any clothes on,” he says.  

In the town of Nipomo on the central coast of California, in Bill Waycott's fields, lettuces of the future are taking root. Red-furled or dark-leaved, these rosettes of crunchy goodness might someday make it to a store near you. 

Waycott is an independent plant breeder, one of the small community of dedicated folks who devote their time and effort to the reproduction of a given plant or plants. For decades, he's focused mainly on breeding better lettuces, and he spoke to TIME about what could be in America's salad bowls in coming years and just how damaging the recent Cyclospora outbreak, traced to a farm run by greens behemoth Taylor Farms in Mexico, has been for lettuce growers.

TIME: How did you intertwine your fate with that of lettuce?

Waycott: My introduction to agriculture occurred as a Peace Corps volunteer in India in the ‘70s. When I came back from that assignment, I wanted to dedicate myself to agriculture. It was the most amazing thing that could ever have happened to me—a California surfer goes to India and learns how to survive in a language, in a culture, in a food system that I had no clue about. I got a job at the [U.S. Department of Agriculture] and the first year and a half it was a sugar beet breeding project. When the lettuce position opened up, I jumped on it.

We were trying to slow down the process of early flowering in lettuce. You probably know that it doesn't last very long in a garden during warm temperatures. It goes to seed pretty quickly. But then we had George Bush Senior come across and clear the deck. I was laid off at that point with a lot of my colleagues back in the '90s, and that's when I went to work for a private breeding company.

In your decades of making lettuce better, what’s been the main focus? 

Well, you know it's 96% water to start with, so you don't have a lot to work with. My focus has been as much as possible on disease resistance—developing lettuces that can stand up to plant diseases that can wipe out a crop—and improved nutrition. Since so many people eat salads these days, even if we were able to improve the nutrition by 5 or 10%, that's a huge impact on the North American diet.

At Nipomo Seeds, which is a company my wife and I started in 2014, we’re working on a lettuce we have nicknamed “Spinach Lettuce,” because it resembles spinach both in the way it grows and also its taste (a softer, spongier, and slightly bitter eating experience). The nutritional values for most of the important nutritional components in lettuce are high in the Spinach Lettuce, especially when compared to iceberg; there’s a lot more fiber, protein, calcium, iron, vitamin C, and vitamin A.

There are others; we have some new lettuces that we're hoping growers will take an interest in. We've been growing field trials, and Chef Dan Barber from Row 7 Seeds has come out to California and looked at them. I think in September we're going to have somebody from Whole Foods come over and have a look.

What’s the biggest challenge facing lettuce breeders these days?

I have to say, I am just flabbergasted by the number of diseases that have come in in the last five to 10 years, and the rapidity with which they're mutating and forming new races. Example: I have been testing for plants that can resist Fusarium oxysporum, which is a fungus of the roots and vascular tissue. The plant's growing along, it's getting close to harvest, and then it just sort of wilts. Pull up the roots, and you see this discoloration. I really did my best to come up with resistance to that issue over the last five or so years. But in the last two years, the disease has changed and formed a new what we call variant race. So all the seed that I produced for that particular issue is no longer interesting to my customers because it doesn't have the brand-new race resistance.

Right now we have three fungi, one bacterium, and one virus here in California lettuce that are wreaking havoc and making life very difficult. In the past, there was one major issue at a time that we could hopefully solve in five years of breeding. Now I'm working on five issues, six issues in tandem. 

How has the Cyclospora outbreak, linked to Taylor Farms’ operations in Mexico, affected the lettuce industry?

Oh, it's disastrous. I've heard sales are at least 30% down, and not just in lettuce. Anything that's green is suspect to some people. If you can't cook it, or buy it out of a can, it’s not on the list. There are fields of mature lettuces being plowed under constantly now that have no value because they can't sell. 

But this isn’t the first time. In 2006, there was a big E. coli outbreak with spinach. That started the whole system of trying to evaluate the product before it was cut. Nowadays, at least north of the border, there are companies that go into every field and sample methodically across the field. Maybe every 30th row or every 50th row, they sample and send these samples into a lab, and it's checked for pathogens. 

Is there a safer way to grow lettuce? Hydroponically, with filtered water?

The hydroponic systems are so expensive—if they were really going to change things, it would have happened by now. 

But there isn't really that much difference; the plant is exposed to the atmosphere, and whatever's in the atmosphere is going to somehow affect the plant. Open field production is the cheapest way to go. Using water, either from wells or from canals, is really the only system available. And when you're looking at production the size of what North America requires, there's nowhere [big enough where] you can filter that water and create a sterile environment for the plant to grow in. It's impossible. But you can at least sample all those components that are going into it, and try your best to avoid any major contamination. 

What are you looking forward to in the future of lettuce?

I hope that we can slowly get to the point where we're improving nutrition. The world right now is pretty chaotic. How does diet fit into all that? How do people make decisions about what they're going to eat? What is the ratio of home cooking to restaurant dining, and financial stability—how does that influence the diet that they choose to use? It is tough. So working on making things more nutritious is an important factor. Hopefully, the growers and the shippers and the marketers agree, and we will bring healthier food into the system, so that people don't even have to decide at the moment of purchase if they should or not, because it's already more nutritious.