Using your numbers, I get the same thing: http://www.wolframalpha.com/input/?i=(10,000,000+m3)+*+(0.15+*+231+micrograms%2Fm%5E3) http://www.wolframalpha.com/inp

Using your numbers, I get the same thing: http://www.wolframalpha.com/input/?i=(10,000,000+m3)+*+(0.15+*+231+micrograms%2Fm%5E3) http://www.wolframalpha.com/input/?i=(10,000,000+m3)+*+(0.15... The microgram is a very small unit :) I mean, a gram is already pretty dinky, a millionth of it is even tinier! The whole point of these particles is that they're miniscule and can pass through tissue other particles can't, so maybe it's not so surprising that it's not a massive amount when you pile them up.