Carbon monoxide poisoning from faulty gas appliances has claimed two lives in Sussex within weeks.
Another seven people survived suspected poisonings in Sussex during recent cold weather, prompting safety warnings from the Health and Safety Executive (HSE).
A 68-year-old pensioner from Hastings was found dead on November 24 after being overcome by fumes from a faulty boiler.
Just weeks later, a 77-year-old pensioner from Crawley was found dead in similar circumstances on Christmas Day.
Andrew Christian, of the HSE at East Grinstead, said: "You can't see, taste or smell carbon monoxide but it can kill. You are particularly vulnerable when asleep.
"Early indicators include tiredness, drowsiness, headaches or pains in the chest and stomach."
Each year, on average, more than 30 Britons die from the effects of the gas, with a further 100 affected.
Mr Christian said: "Owner-occupiers are strongly advised to have their gas appliances checked for safety annually by a Corgi-registered installer."
He said landlords also had a duty to maintain appliances.
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