Rumors of dangerous cosmic rays passing by Earth "tonight," largely identical in their phrasing, have been steadily collecting in our inbox for well over a year. The warning, which has been around since at least 2014, and whose timelessness is assured by the lack of specific dates or a discussion of time zones, is generally phrased as such:
VERY URGENT! Tonight at 00:30 to 03:30am make sure to turn off your phone, cellular, tablet etc & put far away from your body! Singapore TV announced on the news! Please tell your family & friends! Tonight 12:30pm to 3:30am for our Planet will be very high radiation! Cosmic rays will pass close to Earth, So please turn off your cell phone! Do not leave your device close to your body, it can cause you terrible damage! Check Google & NASA BBC News! Send this message to all the people who matter to you! Thank you
Equally imprecise is the science involved in the warning. When astronomers discuss cosmic rays, they are almost always referring to high energy protons from outside our solar system that travel at nearly the speed of light and are thought to be ejected from supernovae, the explosion of stars. Sometimes, however, the term also includes high-energy particles from the sun, as described by Caltech astronomer Richard Mewaldt:
Cosmic rays are high energy charged particles, originating in outer space, that travel at nearly the speed of light and strike the Earth from all directions. Most cosmic rays are the nuclei of atoms, ranging from the lightest to the heaviest elements in the periodic table. [...] The term "cosmic rays" usually refers to galactic cosmic rays, which originate in sources outside the solar system, distributed throughout our Milky Way galaxy.
However, this term has also come to include other classes of energetic particles in space, including nuclei and electrons accelerated in association with energetic events on the Sun (called solar energetic particles), and particles accelerated in interplanetary space.
The National Aeronautics and Space Administration (NASA) and the National Oceanic and Atmospheric Administration (NOAA) (among other agencies) constantly monitor the sun for events that would eject solar particles in the direction of Earth. They do this not because of the health risk they pose to humans (unless you are an astronaut in space) but because of the risk they can pose to electrical grids and devices, as described by NASA:
When struck by [...] a coronal mass ejection, our planet's magnetic fields jostle back and forth. This generates electric currents, radio waves, and accelerates particles. As the atmosphere changes, GPS satellite frequencies that must travel through the ionosphere can be disrupted, resulting in errors of [...] a couple of yards. For airlines, military operations, farmers' vehicles, and financial transactions that rely on GPS, this interference can prove damaging.
Another type of eruption from the sun, called a solar flare, can interfere with shortwave radios. These low frequency radio waves use the ionosphere as a mirror to reflect transmissions around the globe; but during a solar storm, they simply disappear up into the sky—unable to bounce off of an atmosphere so changed by these storms.
These events would cause no direct health risk to humans on the surface of Earth, and as such that non-existent risk would not be exacerbated by the presence of a cell phone near you. The idea that NASA would send out a warning to that effect is belied by the fact that not a single warning regarding solar activity from 2010 to 2015 provided any guidance on the placement of your cell phone during such an event. Additionally, the three hour window for avoiding your cell phone is nonsensical, given the fact that such events generally disrupt our magnetic field for a longer period of time.
If, however, the warning was referring to a more classical definition of cosmic rays — the high energy (mostly) protons that come from supernova explosions — then the warning is on even shakier scientific ground. That's because cosmic radiation, which is bouncing around all over the place and deflected by any magnetic field it runs into, is not easily tied to a single origin. In fact scientists only conclusively proved that cosmic rays generally come from supernovas in 2013, despite suspicions for a century prior. Cosmic radiation is a slowly varying background process that does not necessitate any action on your part.
The only conceivable source of an acute cosmic radiation spike would be a stream of particles ejected directly at Earth from a nearby supernova explosion. In addition to the fact that such an event would conceivably offer decades of warning, and therefore make an exclusive scoop by Singapore TV unlikely, there are also no stars both close enough to us and close enough to the end of their life cycle to produce such an event, according to NASA:
Astronomers estimate that, on average, about one or two supernovae explode each century in our galaxy. But for Earth's ozone layer to experience damage from a supernova, the blast must occur less than 50 light-years away. All of the nearby stars capable of going supernova are much farther than this.
In terms of health effects to humans on the ground, this background cosmic radiation, similar to the solar radiation, poses no immediate harm and cannot be considered a health risk only unless you are chronically exposed to higher radiation levels in polar regions, according to NOAA:
When these particles hit the atmosphere, large showers of secondary particles are created with some even reaching the ground. These particles pose little threat to humans and systems on the ground [...]. The Earth's own magnetic field also works to protect Earth from these particles largely deflecting them away from the equatorial regions but providing little-to-no protection near the polar regions [...]. This constant shower of GCR particles at high latitudes can result in increased radiation exposures for aircrew and passengers at high latitudes and altitudes.
Because this identical warning has been online for years, and because neither interpretation of the warning's text makes scientific sense, we rank this claim false.