We all forget that smoke and gas detectors, are operating 24hrs a day. Even though they are tested and rated for x number of years of operations, they get tired and will stop working within their rated period. Please check their expiry date, date of manufacture or if they have a date at all. If they have an expiry date replace them within 6 months of the expiry date, if they don't have an expiry date, but a date of manufacture replace them within 7 years and if they don't have any date information replace them. If you are using battery units check the date of manufacture, keep replacing the batteries in them or just replace them with the new models that have 10 year lithium battery in them. They cost more, but so do batteries and your time. Remember to hold on to the instructions, do not throw that paper out. When these smart units start to squawk on a Sunday morning you may want to know what they are trying to tell you. Far more important: READ THE INSTRUCTIONS.
The 3rd and 4th gas in the home or cottage is natural gas and propane gas. Commonly used every day with out thought about the consequences if something goes wrong. It happens often enough, but when these go wrong they not only affect the house hold, but your community as well.