It strikes without warning every year around this time, sometimes killing entire families as they sleep.

The worst part is that carbon monoxide poisoning is usually avoidable.

In an effort to prevent such tragedies, Pacific Gas and Electric Co. issued a reminder Monday about the dangers associated with the colorless, tasteless, odorless gas.

Carbon monoxide is released during partial combustion of fossil fuels and wood. It is most dangerous when produced indoors by an improperly ventilated gas appliance, or when a barbecue or other outdoor heating source is brought inside a home.

The symptoms of carbon monoxide poisoning can easily be mistaken for flu: headaches, dizziness, drowsiness, nausea and convulsions.

PG&E, the San Francisco-based gas and electric utility, issued a number of safety suggestions, including inspection of flames on natural gas appliances. It noted that a blue flame indicates complete combustion, while a "lazy, yellow or white" flame signals that the appliance is not burning properly and could be generating carbon monoxide.