One of the major requirements of an illuminated exit sign is it has to keep glowing no matter what. That means that, if power is cut to the building, or if the sign itself gets knocked around, power has to keep flowing. What's the power source? Radiation. But there's no need to panic.
In 1934, three physicists, among them the famous Ernest Rutherford, were smashing deuterium atoms with other deuterium atom nuclei. Deuterium atoms consist of a regular hydrogen atom with a neutron attached. Given the ingredients, the scientists should probably have gotten a lot of helium atoms. Instead, they got tritium – an isotope of hydrogen with two neutrons instead of just one.
Tritium isn't a hard isotope to produce. When cosmic rays go careening through the atmosphere and happen to hit a couple of deuterium atoms, they can produce tritium just the way the physicists did. Since deuterium is rare, it's more common for the cosmic rays to hit nitrogen, which scrambles and recombines with a neutron into tritium. Humans also create tritium. It's a by-product of nuclear reactions.
Tritium can be dangerous, but overall it has probably saved more lives than it has cost. It's what's powering the green glow of emergency exit lights. Tritium decays when one of its neutrons turns into a proton, turning it from a hydrogen isotope into a helium isotope. When the atom does this, it shoots out both an electron and a gamma ray. If the electron should hit a phosphor, the phosphor will glow. Many luminous consumer goods are tubes or screens of glass, coated with phosphors and containing tritium gas.
So what happens when these tubes get broken? It's true that beta particles count as "ionizing radiation," which, in the body, has been known to cause cancer. However, outside of the body, the electrons, or "beta particles," can't get through a sheet of aluminum foil. Beta radiation generally only penetrates the outer layers of skin, but enough of it can burn the new-forming skin underneath the layers of dead cells.
Slightly more worrying than tritium gas is tritiated water. Tritium, when exposed to oxygen, will combine to make H2O. This is a substance that people often put into their body. Fortunately, the tritium doesn't have to stay in the body any longer than the water does, and getting water out of a body can be fun. It wasn't uncommon for scientists at Los Alamos, when working with reactions that produced tritium, to head down to a bar and have a drink. The intake of beer would flush the tritium out of their system.