On July 16, 1945 the nuclear age was ushered in with the test of the first atomic bomb in New Mexico, called the “Trinity Test.” Two nuclear bombs have been used in warfare, but over 2,000 have been tested, including 528 above ground.
The 528 above-ground tests spread radioactive materials through the atmosphere. As discussed in this IFOD, air molecules are jostled around such that after a few years any radioactive airborne particles will be spread throughout the atmosphere. This has caused there to be a tiny bit of radioactivity in the air we breathe. The amount is very small and considered safe.
A side result of the radioactivity of the air is that any steel produced since 1945 also contains a small amount of radioactive material. Usually, that is fine. Who cares if the girder of new office tower or bridge has trace amounts of radioactive material? However, there are some devices which are very sensitive, such as Geiger counters, satellites senors or certain medical devices which require steel without any radioactive properties. Manufacturers of those devices need to either produce steel without radioactivity, which is extremely expensive, or obtain steel produced prior to July 16, 1945. Steel without radioactivity is called “low background steel.”
Common sources of this “low background steel” have been from ships sunk during World War II. There is a pretty robust industry of finding and recovering steel from old sunken ships to be recycled into medical devices and the like. Pretty wild.
Since the 1960s there has been a ban on atmospheric nuclear testing and as a result the radioactive elements in the air have been decreasing because radioactivity decreases with time. The main radioactive isotope in the air is Cobalt-60 and it has a half-life if 5.26 years, meaning the original 1945 Cobalt-60 is a mere .008% as radioactive as its original level. Thus, the problem of slightly radioactive air is going away over time.
Related IFOD: We are all Radioactive