It's nitrogen dioxide, a byproduct of natural gas combustion.
One of the first thread I posted here was an article about it:
http://www.hawthornevillager.com/phpbb/viewtopic.php?f=1&t=47607
Curiously enough, some blasted me for it. From which I roughly gauged the level of retard on this board
[Not that bad]Anyway,
One addition I'd like to add to fm1010's comment; The levels being put out by the plant are safe, however NOx (Nitrogen Oxide) gases can become toxic in large enough concentration.
The exhaust monitoring is performed by the plant's operator, the exhaust data is than submitted to Environment Canada and can be reviewed on the sites I linked in my original article.