As technology improves, homes are being built to be increasingly air tight to prevent expensively heated air from seeping out of your home around door frames and windows. A few years ago, enough air seeped into homes from the outside to replenish the air lost through the combustion process of burning wood or gas. Now, however, not enough air is able to enter the home which may create negative pressure which can cause all manner of problems from smoking issues to drafting problems.
With these new super tight homes, once air is back drafted down through the chimney, it along with any toxins it may bring in from the chimney smoke byproducts are trapped in the home. This can make for a very unsafe condition, which is why it is so important to have an outside air source.
Sometimes, merely opening the window closest to the firebox just a crack is enough to make up for the oxygen being depleted by combustion. This allows enough air to enter the home to prevent negative pressure from occurring.
You may also want to consider installing an outside air source directly into the firebox. A vent is installed such that the combustion process can draw in new air directly from the outside when it is needed, reducing the likelihood that any air will be sucked out of the home.