It comes just days after radiation readings were found to be double the normal amount in Missouri’s recent snowfall.

According to YouTube user, DutchSinse, who posted a video of him taking the Geiger readings in St Louis, the findings mean that ‘small particles of radioactive material are indeed coming down in the precipitation. Past tests show around 30CPM in the same spot on a nice day with no precipitation’.

He said snowstorms in 2012 also showed alert level radiation readings.

The San Francisco findings prompted federal officials to launch an investigation, according to Infowars.com.

Health officials in San Mateo County confirmed the spike – which was reaching over 150 micro-REM per hour – but said they were ‘befuddled’ about what had caused it.