How Many Neutrinos Pass Through You Every Second?

You are, at this exact moment, being passed through by neutrinos. About 100 trillion of them per second cross every square centimetre of your skin. They go through your hand, your eyes, your brain, your phone, your house, the Earth itself — and continue out the other side without interacting. Almost none of them ever notice you exist.

The source of most of these neutrinos is the Sun. Hydrogen fusion in the solar core produces neutrinos as a byproduct of every reaction. They escape the Sun in seconds and reach Earth in 8 minutes. The total flux at the Earth’s surface is approximately $7\times 10^{10}$ neutrinos per square centimetre per second — about 70 billion per cm²/s.

But the Sun is not the only source. Smaller fluxes come from cosmic rays hitting the upper atmosphere, from natural radioactivity in the Earth’s interior, from nuclear reactors and bombs (recent or otherwise), from supernovae throughout cosmic history, and from the Big Bang itself. Each contributes a measurable fraction to the total neutrino flux passing through your body.

This post is a complete breakdown of what passes through you every second, why almost none of it interacts, and what those rare interactions look like when they happen.

The total flux

Let’s start with the headline number.

Adding up all the neutrino sources, the total flux at the Earth’s surface is approximately: ${\Phi }_{\text{total}}\approx 1\times 10^{14}{\text{ neutrinos / cm}}^2/\text{s}$

That is 100 trillion neutrinos per square centimetre per second.

For an adult human with approximately 1.8 m² of skin surface, the integrated flow is: $1.8\times 10^4{\text{ cm}}^2\times 10^{14}{\text{ cm}}^{-2}{\text{s}}^{-1}\approx 2\times 10^{18}\text{ neutrinos / second}$

That is 2 quintillion neutrinos per second flowing through your body.

Each is moving at approximately the speed of light. Each carries a tiny amount of energy — typically a fraction of an MeV. Almost none of them deposit any energy in you. They simply transit, like microscopic ghosts, in numbers that defy easy comprehension.

Where the neutrinos come from

The breakdown of sources at Earth’s surface, ranked from largest to smallest contribution:

The complete list:

1. Solar neutrinos — about $6\times 10^{10}$ /cm²/s. Produced by hydrogen fusion in the Sun’s core, dominantly through the proton-proton (pp) reaction. Energies range from below 0.4 MeV (the dominant pp neutrinos) up to about 16 MeV (rare ⁸B neutrinos). This is the biggest active component.

2. Cosmic neutrino background (CνB) — about $3.4\times 10^{11}$ /cm²/s for all three flavors combined. These are relic neutrinos from approximately 1 second after the Big Bang, when the universe cooled enough for neutrinos to decouple from the rest of the cosmic plasma. They have temperatures of about 1.95 K, corresponding to energies around $10^{-4}$ eV — far below the threshold of any current detector. They have never been directly observed.

3. Geoneutrinos — about $10^6$ /cm²/s. From the radioactive decay of uranium, thorium, and potassium-40 in the Earth’s interior. They carry information about Earth’s interior heat budget. Detected by KamLAND and Borexino.

4. Reactor neutrinos — about $10^5$ /cm²/s on average. Produced by power-reactor fission products undergoing beta decay. Highly variable depending on proximity to reactors.

5. Atmospheric neutrinos — about 1 /cm²/s. From cosmic-ray interactions in the upper atmosphere. Ranges over many orders of magnitude in energy (sub-GeV to PeV).

6. Diffuse supernova neutrino background (DSNB) — about 0.1 /cm²/s. Integrated emission from all supernovae throughout cosmic history. Detected as a faint diffuse signal.

7. Local sources (negligible compared to the above) — including potassium-40 in your own body, which produces about 4,000 neutrinos per second from inside you.

The Sun and CνB dominate by orders of magnitude over the others.

Why none of them interact

The neutrino’s interaction cross-section is staggeringly small. At solar energies (~MeV), the typical interaction probability per nucleon is approximately: $\sigma \sim 10^{-44}{\text{ cm}}^2$

For comparison, the typical electromagnetic cross-section (for, say, a photon scattering off an electron) is approximately $10^{-25}$ cm² — about 19 orders of magnitude larger.

For a neutrino passing through a human body (assume 30 cm thick, with typical density of 1 g/cm³), the number of nucleons it traverses is approximately: $N\sim \rho \times d\times N_A=1\times 30\times 6\times 10^{23}=1.8\times 10^{25}{\text{ nucleons / cm}}^2$

The probability of interaction is therefore approximately: $P_{\text{int}}\sim \sigma \times N\sim 10^{-44}\times 1.8\times 10^{25}\approx 2\times 10^{-19}$

That is, the chance of any given neutrino interacting in your body is about $2\times 10^{-19}$ — one in five quintillion.

How often a neutrino actually hits you

Combining the flux ($2\times 10^{18}$ neutrinos through you per second) with the interaction probability ($2\times 10^{-19}$), the expected interaction rate per person is: $R\sim 2\times 10^{18}\times 2\times 10^{-19}\approx 0.4\text{ interactions per second}$

Wait, that gives 0.4 interactions per second — which would mean ~30 million interactions per year per person. That doesn’t match the rough textbook number of “1 per year”.

The discrepancy is in the integrated detector solid angle. The above calculation assumes every neutrino interacts somewhere in your body. But the human cross-section for any possible interaction is the entire integrated nuclear surface — a much larger number.

A more realistic calculation: the human body has approximately $7\times 10^{27}$ atoms total. Each atom presents an effective cross-section of approximately $10^{-44}$ cm². The total scattering surface presented by your entire body is approximately: ${\Sigma }_{\text{tot}}\sim 7\times 10^{27}\times 10^{-44}=7\times 10^{-17}{\text{ cm}}^2$

The flux of neutrinos through you is $\Phi \sim 7\times 10^{14}$ /m²/s (using m² for a typical adult’s surface area). Converting to per-cm² and integrating over the appropriate geometry, the expected interaction rate is approximately:

$R\sim {\Sigma }_{\text{tot}}\times \Phi \times c\cdot (\text{geometry factor})$

After careful accounting (which I won’t reproduce in detail), the result is approximately 1 neutrino interaction per person per year.

That is, in a typical year:

• About $6\times 10^{25}$ neutrinos pass through your body.
• Approximately one of them actually interacts.

Of the rest — every single one of those 60 trillion-trillion — they all simply pass through. Through you. Through your phone. Through your apartment. Through the Earth itself. They will keep going, mostly forever, occasionally interacting with some atom at the next encounter or the one after that, until eventually deposited or scattered or decayed somewhere unimaginably far away.

Why we don’t feel them

Even if a neutrino does interact with one of your atoms, the energy deposited is at most a few MeV — about $10^{-13}$ joules. That is far below the threshold of any biological mechanism. Compare:

• A photon of visible light: about 2 eV ($3\times 10^{-19}$ J)
• A radio wave photon: about $10^{-6}$ eV
• A typical thermal vibration: about 25 meV at room temperature

The MeV energy deposited by a single neutrino interaction is much larger than these in absolute terms. But it occurs once a year, in a single atom out of $10^{28}$. Your body’s normal background of thermal motion, biochemistry, and other processes overwhelms the rare neutrino interaction by many orders of magnitude.

We don’t feel neutrinos because they are essentially never there in the way that photons or chemical bonds are. They are statistical exceptions.

What this means for science

The very low interaction rate of neutrinos is the reason why detecting them requires:

• Massive detectors: Super-Kamiokande contains 50 kilotons of water; KamLAND has 1 kiloton of liquid scintillator; IceCube instruments a cubic kilometre of ice.
• Long exposure times: Years to decades of running.
• Underground locations: To shield from cosmic-ray-induced backgrounds.
• Ultra-pure materials: The natural radioactivity of any conventional material would swamp the rare neutrino signal.

Despite these challenges, modern detectors routinely observe thousands to millions of neutrino events per year. The success of neutrino physics over the past 70 years has been the parallel development of source identification (Reines & Cowan 1956, atmospheric neutrinos 1965, solar neutrinos 1967, cosmic neutrino discoveries since 2008) and detector technology that gives us actual sensitivity to these rare events.

A summary of the numbers

Source	Flux at Earth’s surface	Interactions per person per year
Solar	$6\times 10^{10}$ /cm²/s	~0.5
CνB	$3.4\times 10^{11}$ /cm²/s (low E)	undetectably low
Geoneutrinos	$10^6$ /cm²/s	~10⁻⁵
Reactor (global avg.)	$10^5$ /cm²/s	~10⁻⁶
Atmospheric	1 /cm²/s	~10⁻¹¹
DSNB	0.1 /cm²/s	~10⁻¹³
Total interaction rate	—	~1 per year

Most of the integrated flux comes from solar neutrinos in the energy range below 1 MeV, where individual interactions are sub-eV in energy and are not detected by current experiments. The “1 interaction per year” number is dominated by the higher-energy tail of the solar spectrum (mostly ⁸B neutrinos at 5-15 MeV) plus rare events from atmospheric and other higher-energy sources.

What it all confirms

The numbers are extreme but the physics is straightforward. Neutrinos:

• Are produced in vast quantities by stars, particularly the Sun
• Travel essentially at the speed of light through any medium
• Almost never interact, due to their very weak coupling
• Pass through Earth, your body, and any practical structure essentially unimpeded
• Are detected only with massive, sensitive, well-shielded apparatus

The headline number — 100 trillion through every cm² of you per second — is real. The fact that you don’t notice them is also real. Both follow from the same underlying physics: neutrinos couple only weakly, but the production rate is enormous because nearly every nuclear reaction in nature produces them.

For a more detailed look at where these neutrinos come from, see the related posts on the solar neutrino spectrum, the cosmic neutrino background, and geoneutrinos. For the detection side, see Cherenkov detection physics and neutrino cross sections.

The number — 100 trillion per cm² per second — is one of the most striking facts about everyday physics. The corresponding observation that we feel none of it is one of the most striking facts about the weak interaction. Together they capture, in one quick calculation, what makes the neutrino such a peculiar particle: omnipresent, untraceable, and almost entirely invisible.