Newsroom
Edited by Eleonora Maria Vladimirou, Well.cy
Every second, countless invisible particles pass through the human body without us feeling a thing. Some come from the Sun, others from distant galaxies, and some form in Earth’s atmosphere. What does science know about this constant “rain” from space, and how much can it affect our health?
We may not notice it, but our bodies stay in constant contact with an invisible world of subatomic particles. Even right now, while you read these lines, an incredible number of particles stream through you and keep going without leaving any noticeable mark. Some started out from the Sun. Others have traveled millions or even billions of years from far-off parts of the Universe.
It sounds striking, maybe even a little unsettling. Still, these particles are nothing new. Earth has faced cosmic radiation for billions of years, and its atmosphere works as a strong natural shield.
To get a clear picture of what happens, we need to look back at a key scientific discovery.
The discovery that revealed invisible radiation from space
In 1912, Austrian physicist Victor Hess ran a groundbreaking experiment that changed how scientists thought about radiation. He rose in a balloon to about 5.3 kilometers and measured the ionization of the atmosphere.
Ionization is the process that gives atoms or molecules an electric charge when they lose or gain electrons. Until then, most people believed a large share of the radiation that caused ionization came from radioactive materials in the ground.
Hess saw something unexpected. As the balloon climbed higher, the ionization did not drop. It rose sharply. At high altitude it reached roughly three times the level measured near sea level.
The finding was revolutionary: a major source of the radiation came from outside Earth. Hess helped discover cosmic rays and opened a whole new path in astrophysics and the study of elementary particles.
More than a century later, we know far more about these particles and their path toward our planet. Earth’s atmosphere acts like a multi-layered filter. Some particles stop in the upper layers. Others collide with atoms and molecules in the air and create new secondary particles. A portion of those reach the surface and pass through our bodies.
What particles pass through our bodies?
1. Neutrinos: Invisible travelers of the Universe
Neutrinos rank among the most common particles that stream through the human body every day. They are subatomic particles with extremely small mass and no electric charge. One of their most striking traits is that they almost never interact with matter. They can travel through huge amounts of material without hitting other particles.
Michael Pravica, a physics professor at the University of Nevada, Las Vegas, told Live Science that roughly 100 trillion neutrinos pass through every person each second. The number sounds unbelievable, yet most of them keep going without any interaction with our bodies.
Most neutrinos that reach Earth form inside the Sun during the nuclear fusion reactions that power it. Some come from much farther away. Lu Lu, an assistant professor of physics at the University of Wisconsin–Madison, explains that certain neutrinos originate in astrophysical events in other galaxies. Studying them holds great scientific value because they can carry information from regions of the Universe where light struggles to escape.
2. Cosmic rays: Particles that travel nearly as fast as light
Another group of particles that reach Earth are cosmic rays. Despite the name, they are not mainly beams of light. They are high-energy particles. Most consist of protons (hydrogen nuclei) along with helium nuclei and nuclei of heavier elements. Smaller numbers include electrons and positrons, the antiparticles of electrons.
Many cosmic rays come from extremely energetic events outside our solar system. Possible sources include supernova explosions, active galactic nuclei, and areas near supermassive black holes. Collisions and interactions between galaxies can also create conditions that accelerate particles.
These particles gain enormous energy and race through space at speeds close to the speed of light. When they reach Earth, many smash into molecules in the atmosphere and set off chain reactions.
3. Muons and neutrons: Particles created in the atmosphere
When a high-energy cosmic ray hits an atom in the upper atmosphere, it can trigger a cascade of secondary particles. Muons and neutrons are among them.
Neutrons are electrically neutral particles that, together with protons, form atomic nuclei. Muons resemble electrons but weigh about 207 times more. They carry an electric charge and can interact with matter as they pass through. Special instruments such as a cloud chamber can track their paths, where charged particles leave visible trails in supersaturated vapor.
Lu Lu notes that at sea level about one muon passes through every square centimeter each minute. For a human body that works out to tens or hundreds of muons every second, depending on surface area and orientation. Roughly one or two muons can cross through a hand every second. Yet we feel nothing at all.
Can these particles affect our health?
Just because particles from space constantly stream through us does not mean we face real danger. Scientists say any biological effect depends on the type of particle, the energy it carries, how often it interacts, and how much radiation the body absorbs.
Why neutrinos pose almost no risk
Under normal conditions on Earth, neutrinos present essentially no health threat. Trillions pass through us every second, but the chance they will interact with any atom in the body is extremely small. Lu Lu points out that ordinary neutrinos that travel through us might produce one or only a few interactions over an entire lifetime. The energy involved in those rare events is so tiny that neutrinos are not viewed as a danger to human health.
What about muons and neutrons?
Unlike neutrinos, muons and neutrons interact more readily with matter. They form part of the natural ionizing radiation we encounter every day. At ground level, muons make up the largest share of the penetrating charged particles that come from cosmic rays, Lu explains.
For an average person, yearly exposure to cosmic radiation runs about 0.4 millisievert (mSv), though the figure changes with local conditions and geography. That amount sits in the same general range as the total dose from a few ordinary chest X-rays, depending on the method used.
The total natural radiation a person receives from the environment averages about 2.4 mSv per year worldwide, though newer estimates and local readings can differ. Natural radiation comes from rocks, soil, radon, certain natural radioisotopes inside the body, and cosmic rays.
At typical levels on Earth’s surface, cosmic radiation forms a relatively small part of overall natural exposure and does not raise special concern for most people.
Why exposure rises on airplanes
The atmosphere shields the planet by absorbing or limiting much of the cosmic radiation. The higher you go, the thinner the protective layer above you becomes. That is why a flight exposes passengers to more cosmic radiation than time spent on the ground.
Geographic latitude also matters because Earth’s magnetic field shapes the paths of charged cosmic particles. Flights at high latitudes, especially near the poles, can bring higher exposure.
For occasional travelers the extra dose from a normal flight stays small. Pilots and cabin crew, however, face higher cumulative doses because they spend many hours each year at altitude. Some epidemiological studies have found higher rates of certain cancers among flight crews. Still, it is not always clear that cosmic radiation alone drives the increase. Other factors such as disrupted sleep cycles, working conditions, and ultraviolet exposure may also play a role.
How cosmic particles help scientists understand the Universe
Beyond radiation questions, the particles that reach Earth supply valuable scientific information. For decades before modern particle accelerators existed, cosmic rays served as a natural laboratory for physicists. Their study led to the discovery of important elementary particles. In 1932 researchers found the positron, the first antiparticle observed in experiments. In 1937 they found the muon. Those discoveries advanced particle physics and our understanding of matter’s structure.
Today one of the most active research areas involves high-energy neutrinos. Because neutrinos interact so little with matter, they can escape regions of the Universe that block light. That makes them especially useful for studying extreme astrophysical events. Lu Lu explains that high-energy neutrinos can deliver information about some of the most violent and hard-to-reach places in the Universe, including zones near black holes, stellar explosions, and other powerful cosmic accelerators.
In short, neutrinos can carry news from events happening at vast distances that we could not study as effectively by watching light alone.
From space to the pyramids: Unexpected uses for muons
Cosmic particles prove useful far beyond astrophysics. Scientists have created methods that use muons to examine the inside of large structures without damaging or entering them. One such method is muon tomography.
The technique measures how many muons pass through an object or building. Density and thickness of the materials change the number of particles that get through. Researchers can then build images of the interior.
The approach has even helped archaeologists. It contributed to finding unknown empty spaces inside Egyptian pyramids. One well-known case involved the Great Pyramid of Giza, where muon detection revealed previously unknown chambers. Similar methods can study volcanoes, geological formations, and large industrial sites.
The same particles born from cosmic collisions in the atmosphere become tools for exploring both space and our own planet.
Should this invisible “rain” worry us?
The thought that countless particles constantly stream through our bodies can feel unsettling at first. Yet it is a completely natural process that happens everywhere on the planet without pause.
Neutrinos pass through matter almost freely and pose no practical health risk. Muons and other secondary particles add to the natural background radiation around us, but ordinary exposure at ground level stays relatively low.
At the same time, studying these particles has produced major scientific advances, from identifying new elementary particles to probing black holes and uncovering hidden spaces in ancient monuments.
Most striking of all, even when we stand still our bodies stay in continuous contact with particles that travel across the entire Universe. It is a quiet reminder that Earth is not isolated. It belongs to a vast, ever-changing cosmic environment.
