Every year around 200 people in Halifax call the National Gas Emergency Service to report suspect carbon monoxide in their homes.

Emergency firm Northern Gas Network, which owns all the gas mains in Yorkshire, and United Utilities, which carried out its day-to-day work, have issued a warning to tie in with Carbon Monoxide Awareness Week.

Engineer Steve Hogarth, who manages the region's gas emergency service, said carbon monoxide was a killer, but was easy to avoid.

"The tragedy of carbon monoxide is that it is so easy to avoid. Simply get your gas, solid fuel or oil appliances serviced by a properly qualified engineer every year," he said.

"Carbon monoxide is a colourless, odourless gas which is produced when fuels such as gas are burned without an adequate air supply. The symptoms of poisoning are similar to flu but the effects are potentially lethal."

For gas appliances, engineers must be registered with CORGI, the Council for Registered Gas Installers.

Mr Hogarth said: "Tell-tale signs that your gas appliance is not working correctly are if the flames, which usually burn blue, are actually burning yellow. Also, if you see there are sooty marks on your appliance."

If in any doubt, turn off the appliance, open the windows and call the National Gas Emergency Service on 0800 111999.