Katherine Heigl is in trouble with the P.C. police. A week after telling David Letterman how she managed to quit smoking by switching to e-cigarettes, the “Life as We Know It” actress is drawing not praise but condemnation from moralistic public health types.

An e-cigarette, for those not in the know, consists of a tube that looks very much like a real cigarette, but is made up of a battery, an atomizer and a nicotine cartridge.

It’s inhaled cigarette smoke that causes so many smoking-related diseases — but e-cigarettes work by vaporizing the liquid nicotine, not burning tobacco.

“You feel like you’re smoking,” Heigl told Letterman. “You get the habit of this” — bringing her hand to her mouth — “and blow out water vapor so you’re not harming anyone around you and you’re not harming yourself.”

So why would anyone be against this innovative device?

Electronic cigarettes “are not a good way or an approved way to quit smoking,” intones Dr. Jonathan Whiteson, medical director of the cardiac wellness and rehabilitation program at NYU Langone Medical Center. “They have never been proven to be effective at smoking cessation.”

And yet e-cigarettes have proven to be 100 percent effective in helping Katherine Heigl quit. She says she’s been using them for six months now. “I know it’s ridiculous, but it’s helping me not to actually smoke real cigarettes,” Heigl told Parade magazine.

Dr. Whitestone apparently would prefer Heigl stick to the “approved” ways to quit smoking — but before trying e-cigarettes the actress had already tried the nicotine patch, nicotine gum and the prescription drug Chantix — twice.

The truth is, the “approved” methods of smoking cessation have a dismal track record, with success rates less than 15 percent after one year. That’s why 1 in 5 U.S. adults are still smoking, a percentage the Centers for the Disease Control and Prevention said last month hasn’t budged since 2005, despite ever-more aggressive smoking bans and higher and higher taxes. We desperately need better options.

E-cigarette opponents argue that the faux cigarettes haven’t been subject to formal safety tests. “Inhaling fumes from the plastic tubes can be carcinogenic,” Dr. Len Horovitz of Lenox Hill Hospital told the New York Daily News.

But would Horovitz really prefer that Heigl and the other 47 million addicted Americans stick to a known carcinogen — one that causes 446,000 deaths each year — rather than try something new and different?

Again, it’s not the nicotine that’s so dangerous about regular cigarettes, but the toxins and carcinogens in the “products of combustion” — the smoke — that’s inhaled deep into the lungs and then into the general circulation. There’s no reason to think e-cigarettes present the same risks, since there’s no combustion.

And while there’s been no formal study showing they can help people quit, anyone who spends much time among “vapers” will hear heartfelt tales of how these products have changed people’s lives by enabling them to beat lifelong addictions. Heigl’s story is one of many.

Cigarette smoking remains the No. 1 cause of preventable disease and death in this country. Alternatives for addicted smokers, such as e-cigarettes and smokeless tobacco products, need to be taken seriously. Heigl and her e-cigarette habit really aren’t all that ridiculous after all — and what’s outrageous is condemning millions of Americans to a premature, lingering death.

Dr. Elizabeth M. Whelan is president of the American Council on Science and Health.