Here is a good source to help explain it for you. Remember that anything is better than nothing so if you want to buy something, anything you can afford is better than nothing.
First you need to know that the elevated radiation is really present and for how long, this is why a geiger counter and outside radiation station (I recommend a HEPA air purifier setup like mine) is so important.
https://www.osha.gov/dts/shib/respiratory_protection_bulletin_2011.html