The people of Orbis has mass teleportation capable of moving entire armies across the world and the ability to summon new taxpaying citizens fully grown the moment they build new houses and you choose to question how they manage to turn 1 ton of steel into a 30 ton tank?
Nope, the city you have the subway in has 58 pollution which, assuming the wiki is correct, is 45 subtracted from 103. 103 being the base amount of pollution if you add all your improvements up.
Or you're looking at the commerce rate instead of the pollution.
It doesn't exist. The Meltdown is just a relic idea that Sheepy never implemented.
Also:
http://politicsandwar.com/forums/index.php?/topic/3332-nuclear-meltdown/
http://politicsandwar.com/forums/index.php?/topic/3632-nuclear-meltdown/
So, apparently the wiki lists two formulas:
http://politicsandwar.wikia.com/wiki/Disease
http://politicsandwar.wikia.com/wiki/City
One of these were correct, though.