Airplane crews at high altitude are exposed to potentially damaging levels of radiation from cosmic rays, studies find. Adrian Melott, professor of physics and astronomy at the University of Kansas, says:

“Neutrons which don’t reach the ground do reach airline altitude. Flight crews get a lot more radiation dose from neutrons. In fact, during solar particle events, airplanes are diverted away from the North Pole, where a lot more cosmic rays come down.”

But might these cosmic rays presnt a hazard even at down at sea level?

Research has suggested in recent years that congenital birth defects on Earth’s surface could be triggered by these “solar particle events”, spikes in cosmic rays from the sun that touch off the northern lights and sometimes impede communications or electric power grids.

But a new NASA-funded investigation, published in the Journal of Geophysical Research, has found radiation from solar events is too weak to cause concern at ground level.

“We looked at two different studies,” said co-author Melott. “Both of them indicated a connection between cosmic rays and the rate of birth defects. One also associated mutations in cells growing in a petri dish with a 1989 solar particle event.”

Melott and colleagues Andrew C. Overholt of MidAmerica Nazarene University and Dimitra Atri of the Blue Marble Space Institute of Science calculated the dose of radiation from a solar particle event to be less than a visit to the doctor might necessitate.

“We have a contradiction,” Melott said. “Our estimates suggest that the radiation on the ground from these solar events is very small. And yet the experimental evidence suggests that something is going on that causes birth defects. We don’t understand this, which is good. Something one doesn’t understand is a pointer to an interesting scientific problem.”

The team investigated how cosmic rays from the sun create hazardous “secondaries” by reacting with the Earth’s atmosphere.

“Cosmic rays are mostly protons,” Melott said. “Basically, they are the nuclei of atoms—with all the electrons stripped off. Some come from the sun. Others come from all kinds of violent events all over the universe. Most of the ones that hit the Earth’s atmosphere don’t reach the ground, but they set off ‘air showers’ in which other particles are created, and some of them reach the ground.”

Such air showers present the most serious threat for the health of humans and other biology on the Earth’s surface via “ionizing radiation,” according to Melott.

“Ionizing radiation is any radiation that can tear apart an atom or a molecule. It can affect life in many ways, causing skin cancer, birth defects and other things. Normally, about one-sixth of the penetrating radiation we get down near sea level is from secondaries from cosmic rays.”

Two forms of radiation formed by solar particle events—muons and neutrons were focused on. They found that muons are the most dangerous to biology at the Earth’s surface.

“Muons are a kind of heavy cousin of the electron,” Melott explained. “They’re produced in great abundance by cosmic rays and are responsible for most of the radiation we get on the ground from cosmic rays. Neutrons can do a lot of damage. However, very few of them ever reach the ground. We checked this because some of them do reach the ground. We found that they’re likely responsible for a lot less damage than muons, even during a solar particle event.”

The next step in the investigation should be fine-tuning an understanding of how much exposure to muons DNA can withstand.

Andrew C. Overholt, Adrian L. Melott, Dimitra Atri A link between solar events and congenital malformations: Is ionizing radiation enough to explain it? J. Geophys. Res. Space Physics, 120, 1537–1542. doi: 10.1002/2014JA020681

Illustration: NASA

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