Two Long Island manufacturers are competing to market sensors that help schools lift the fog shielding users of hard-to-detect vaping  devices.

As a new academic year opens, schools on Long Island and elsewhere are installing the sensors amid warnings of an “epidemic” among U.S. youth using vape pens, also known as e-cigarettes, which can deliver nicotine or THC, the psychoactive ingredient in marijuana.

Adam Wandt, a professor at John Jay College of Criminal Justice in Manhattan, said most e-cigarettes do not set off smoke detectors and their stealth tempts users to vape in places “where they would never have lit a cigarette” — including in schools, movie theaters and airplane bathrooms.

“It’s becoming way too easy to use nicotine and drugs without leaving trace evidence,” he said.

The sensor devices from Hauppauge-based Soter Technologies LLC and holding company Advance Convergence Group of Bay Shore can detect vaping in general, but also can specify when THC is being used.