Where the streets are radioactive

n the 1910s and 1920s, Denver was home to a plant that processed radium, largely for medical applications. Due to lax safety standards, radioactive residues ended up in waste that was used to bulk up foundation materials as well as in concrete and asphalt mixes. A subsequent review found that nine streets in Denver had been laid with asphalt contaminated with radioactive residues and peak gamma radiation levels were measured at 57 micro-roentgens per hour - far in excess of the 7 micro-roentgens per hour average US background radiation dose. The investigation concluded that the levels of radiation posed only a minimal threat to public health - even if someone stood at the location of the highest measured level of radiation for 16 hours, they would receive less than the recommended maximum annual safe radiation dose standard. Nonetheless, in the early 2000s, the city spent $20 million to cut out and safely dispose of sections of contaminated asphalt.