Now the Department of Homeland Security is teaming up with high-tech firms to develop a cellphone that could also thwart terrorist attacks.

The department’s science and technology arm is spearheading a plan to give cellphones the ability to sniff out dangerous chemicals. If successful, the phones could help detect chemical attacks at airports, train stations and subway stops, the agency said.

In 2007, the department called on companies to develop the danger-sensing technology. Now it’s pushing ahead, working with wireless technology developer Qualcomm Inc., camera lens specialist Rhevision Technology Inc. and NASA.

As they are envisioned, these cellphones would sound an alarm if they sensed a noxious gas such as chlorine.

But if the phone detected something more deadly, such as sarin gas, it could send a message directly to authorities, using GPS technology to pinpoint the location of the gas.

If the idea works, every person armed with a cellphone could become a sentry against terrorist attacks. Nearly 90% of the U.S. population owns a cellphone.