Look at these crazy facts we just came across comparing the cleanliness of various forms of heat.  The chart below is from burningissues.org and illustrates just how dirty heating sources such as wood boilers, wood stoves, and oil heat are.

If you take a look at the numbers of how much pollution is being produced, it is staggering.  The typical gas furnace produces 0.00005 pounds of pollution an hour.  Compare that to an oil furnace at 0.001 pounds an hour, a pellet stove at 0.127 and all the way up to a wood stove at 0.825!  The numbers show that gas furnaces reduce pollution emissions significantly.