Here’s a challenge to America’s smoke and carbon-monoxide alarm industry: Create one device that can sense flames and smoke and warn you about carbon monoxide. Consumer Reports’ recent tests of 25 alarms show that safety is far too complicated.

For example, smoke alarms that use ionization technology were great at detecting a fast-flaming fire, like burning paper, but lousy at warning you of a smoldering fire, say, in a mattress. The opposite was true of photoelectric versions. A few smoke alarms, which use both technologies, warn you of both types of fire, but they don’t detect leaks of carbon monoxide, a colorless, odorless gas emitted by fuel-burning appliances that can cause brain damage or death. And combined smoke and carbon monoxide alarms detect only one type of fire, depending on the maker.

Another hurdle is getting all of your home’s alarms to communicate with one another via an electrical line or wireless signal, so you would be warned about a fire or carbon monoxide leak in the basement even if you were asleep upstairs. Adapters enable hardwired alarms to be connected with those made by a competitor. But wireless alarms can communicate only with other wireless alarms from the same company, since manufacturers use different frequencies. Consumer Reports believes that the industry should fix this problem.

Fortunately, you can safeguard your household with some mixing and matching. What’s more, several models provide excellent protection. And any of CR’s tested units would be better than having no detectors at all.

CR’s recommendations

For top overall protection, CR recommends the Best Buy dual-sensor Kidde PI2000, $30, smoke alarm, which connects to home wiring or an existing alarm system and has battery backup for blackouts. When paired with the Kidde Silhouette KN-COPF-1 CO detector, $60, they will give the best protection.

The First Alert SA302CN, $25, and the Kidde PI9000, $23, are both recommended as battery-powered smoke detectors and combine both types of smoke sensing in a single battery-powered alarm. For the kitchen and bath, the BRK 7010B, $25, a photoelectric model with battery backup, offers excellent protection against smoldering fires without the risk of nuisance alarms. The First Alert SA9120BCN, $15, is recommended when a hardwired ionization sensor is needed to round out an existing alarm system.

As for carbon monoxide alarms, the First Alert OneLink SCO501CN, $70, a CR Best Buy, combines carbon monoxide and photoelectric smoke detection and can communicate with other wireless First Alert alarms in a large, multi-story home. For a stand-alone model in a small space, CR selected the First Alert CO615, $40, as a Best Buy for its plug-in with battery backup capabilities.

How to choose

CR reminds shoppers that some insurers offer a 5 percent discount if the home has smoke alarms. It recommends that people install both types of smoke sensors and a carbon monoxide alarm on every level and a smoke alarm in the attic, in all bedrooms, and in hallways. CR also suggests that consumers consider the following when choosing alarms:

l Pick your power. Hardwired systems tie into a home’s wiring and require professional installation (about $250 per unit), whereas battery-only alarms and plug-in carbon monoxide alarms are easy do-it-yourself projects.

l Look for key features. Smoke alarms should have a hush button, safer than disabling the alarm by removing the power during a false alarm. On carbon monoxide alarms, look for a digital display showing carbon monoxide levels in parts per million; levels as low as 30 ppm can harm heart patients, pregnant women, and children.

l Balance convenience and cost. First Alert’s SCO501CN, $70, and Kidde’s KN-COSM-1B, $45, combine CO and smoke detection. But each covers only one type of fire and, like all carbon monoxide alarms, should be replaced every five years. Smoke alarms should be replaced every 10 years.