A couple of weeks ago, the IAEA and ISO put forward a brand new symbol to makr areas where the danger of ionizing radiation is present. The new symbol is to supplement the classic black-on-yellow trefoil you so associate with radiation hazards.

Basically, it was felt that the classic symbol on its own did not really carry any semantic meaning: it wasn't clear from looking at it what it meant. The new design has been extensively tested on various groups to ensure that, at just a glance, its meaning is absolutely clear:

If your ceiling fan starts dropping tapeworms on pirates, run like hell.