The U.S. men’s soccer team beat Mexico yesterday. Neither team played a full-strength lineup, and, as an international friendly, the game, literally, counted for nothing.

Some other things: The game was played in Estadio Azteca in Mexico City, where the U.S. was 0-8-1 coming into the game. The Americans were 0-23-1 in all games against Mexico played in Mexico before yesterday.

Whatever this game actually “means” doesn’t matter. But magnitude of meaning notwithstanding, winning a game in Azteca is a rare thing for any team that isn’t Mexico, so rare—Mexico has only lost nine times at Azteca, ever—that, by definition, it is impressive, whatever the circumstances. Especially when you consider that the Americans basically did it with plastic bags on their heads.

MEXICO IS A GOOD-unto-average-unto-good-unto-etc. soccer team, much like the U.S. By no means are they a world power. They’ve never won a World Cup and haven’t ever made it past the quarterfinals. They’ve dominated the youth-soccer circuit over the past few years, so maybe they’re on the verge of some breakthrough, but that still doesn’t explain the past 90-or-so years.

Azteca holds 104,000 people, all of whom, if they care about personal health beyond the next 90-plus minutes, are rooting for Mexico. Azteca’s fans have been know to launch beers, bags of vomit, and ziplocs of urine at players. For their own safety, Mexican police routinely escort American fans out of the stadium. It’s rare for American broadcasters to leave Azteca not covered in alcohol. Plus, you know, it’s not like Mexico is a place that you’d necessarily go if your main trip requirement was “don’t die.”

So, add all that up and it creates an intimidating environment for sure. But still, it’s way too easy of an explanation for such sustained, dominant dominance from a country that’s really not all that dominant on the world-soccer scene. A better—but still not complete—explanation might have something to do with an imaginary plastic bag that’s not filled with human excrement.

FIFA PRESIDENT SEPP BLATTER banned games at high-altitude stadiums a few years ago, but Azteca comes in right on the edge of that threshold, at 7,349 feet. Also, while Mexico City’s historically gross air-pollution levels have improved, they’re still about on par with the levels in Los Angeles, which was given an “F” for air-quality levels by the American Lung Association earlier this year.

“Once you get above 4,500 feet, your body starts to feel the effects of altitude,” said Sarah Baysden, instructor of Exercise and Sports Science at Western State Colorado University. “Seven thousand feet is pretty significant if you’re not used to barometric pressure.”

As anyone who’s ever gone from sea level to a couple thousand feet up knows, it’s kind of a shocking adjustment. It’s harder to get oxygen into your lungs, making it harder to breathe and even harder to catch your breath. And that’s when you’re just walking around, not playing a high-level soccer game for an hour-and-a-half.

It takes about a week to acclimatize to high altitude, getting over any initial illness/shock. And that’s usually the most training-time visitng teams get before playing. But it requires something like 104-times that to actually get to your regular-functioning level.

“It takes usually about two years of living in altitude to become accustomed to it,” Baysden said, “while the Mexican athletes don’t know any different.” Real, scientific home-field advantage.

The air pollution plays a part, too, but that would affect both teams more evenly, according to Baysden. Still, it certainly doesn’t make things any easier for Azteca’s visitors.

So, what was it like for, say, Fabian Johnson, who, coming from Germany, played 90 minutes for the U.S. last night?

“It feels as if your rib cage or diaphragm won’t expand,” Baysden said, “and it feels like you can’t catch your breath. Trying to catch your breath between bursts of speed, you definitely won’t be able to feel like you’re recovering.”

More simply?

“It’s like breathing with a plastic bag on your head."