The count rate of particles vs. the altitude of earth’s atmosphere (hyperphysics.phy-astr.gsu.edu, 2010)
When a primary cosmic ray particle enters earth atmosphere, it will encounter many atoms and collisions will occur. This collisions will send a shower of “secondary” particles which are also subatomic particles such as pions which will quickly decay into relatively stable muons, neutrinos and gamma rays. Muons will soon too decay into electron and positrons which they both will annihilate and the gamma rays will interact with atmospheric atoms.
The numbers of particle that finally reached the earth’s surface depends on the energy content of the "primary" cosmic ray particle. Most secondary cosmic rays that reached the surface of earth are muons with an average intensity of 100 counts per metre square per second.
Although thousands of cosmic ray particles pass through our body every minute, the resulting exposure to that amount of radiation is minimal, safe, and is considered as background. We can say that we are irradiated with fast particles every single moment in our lifetime with negligible health effects.