Traveling the skies by jet lifts us far from the hustle and bustle of the world below.
But many flyers don't know that soaring miles above Earth also takes us out of a vital protective cocoon - and a little closer to a place where our cells can be pummeled by radiation from colliding stars, black holes, and more.
You can't see these high-energy charged particles, but at any given moment, tens of thousands of them are soaring through space and slamming into Earth's atmosphere from all directions.
Also called cosmic rays or cosmic ionizing radiation, the particles are the cores of atoms, such as iron and nickel, moving at nearly light-speed. They can travel for millions of years through space before randomly hitting Earth.
These rays don't pose much of a risk to humans on Earth's surface, since the planet's atmosphere and magnetic field shield us from most of the threat.
"Cosmic rays are not a significant exposure risk on the ground," Eddie Semones, a radiation health officer at NASA, previously told Business Insider. "You actually get more exposure from the Earth's natural radioactive material than from galactic cosmic rays."
But far above the ground, the particles are more likely to leak through.
When cosmic rays strike the air, they create showers of ionizing radiation - particles that can knock electrons free of atoms and molecules - that can penetrate deep inside our bodies. This potential damage to tissues and DNA poses risks to our health and has been linked to cancer, reproductive issues, and cognition problems in animals.
That's why it can be risky to fly for a living. But cosmic rays aren't the only form of radiation that pilots and flight attendants face.
Dangers from deep space
Because those who fly on jet airplanes frequently face higher exposure to cosmic radiation, the Centers for Disease Control and Prevention (CDC) classifies airline crew members as radiation workers.
In fact, the National Council on Radiation Protection and Measurements reported in 2009 that air crews have, on average, the highest yearly dose of radiation out of all radiation-exposed workers in the US. This means they receive more radiation exposure than people who work alongside nuclear reactors.
"Working at power plants nowadays, it's very, very strict in how they would do a job that'd incur radiation exposure to an individual," Semones said. The use of robots and detailed planning has become routine since the Cold War, so "the industry has evolved," he added.
Another radiation risk when flying is energetic outbursts from the sun, which Earth normally shields people from. This includes gamma rays and X-rays from solar flares, and storms of high-energy protons. When the sun is very active, there can be several solar particle events per day.
The annual exposure for air crews is an estimated 3 millisieverts (mSv) - a complex measure of the amount of background radiation a person receives in one year in the US. Only astronauts are more exposed: Ten days in space delivers about 4.3 mSv to the skin alone, which is 4.3 years' worth of cosmic radiation on the surface of Earth.
This is why NASA does not permit astronauts to spend more than a year or so in orbit. The space agency doesn't want their exposure to boost their lifetime risk for cancer more than an additional 3%.
No limits for airline crewsintense exposure
"There are no official dose limits for aircrew in the United States," the US Centers for Disease control writes in its aircrew safety guidelines on cosmic radiation. The reason: "We don't know what levels of cosmic radiation are safe for every person."
This is because there are very few human studies on the topic. Most have examined nuclear bomb survivors and people undergoing radiation therapy. The animal studies that have been done don't always map well to people.
There are some worldwide guidelines, however.
The International Commission on Radiological Protection recommends crew members not be exposed to more than 20 mSv per year. The ICRP says that the general public, on the other hand, should receive less than 1 mSv per year.
To minimize their exposure, airline personnel should try to limit working on flights that are very long, at high altitudes, or that fly over the poles, which are all associated with heightened exposures, according to the CDC.
Pregnant crew members are particularly at risk and should try not to fly during their first trimester. They're also advised to avoid flying when the sun is having a solar particle event, which can deliver a higher dose of radiation in one flight than is recommended for the entirety of a pregnancy.
To calculate your exposure on a typical flight, check out this handy online tool from the Federal Aviation Administration.
Julia Calderone wrote a previous version of this post.