While sending smokers outside to indulge saves others from copping a lungful of carcinogens, growing evidence shows it's not as easy to escape their fumes as we thought.

It's becoming clear that 'third hand' smoke particles clinging to fabrics and other surfaces can continue to cause harm. New research indicates they can easily become airborne again, drifting into areas we'd otherwise consider smoke free.

A study by researchers from Drexel University in the US analysed the air of an unoccupied, smoke-free university classroom.

Except it wasn't as smoke-free as you would expect. The team found compounds that carry the signature of cigarette smoke that's hung around in the environment.

One nearby hot spot of tobacco aerosols included a balcony roughly 20 metres (60 feet) away, where smokers would step outside for a quick puff.

Another potential source in the vicinity was an office space that shared the same heating and air conditioning ducts.

Whichever was the culprit, distance and a few walls clearly wasn't enough to screen potentially toxic particles from the space designated safe from smoke.

"While many public areas have restriction on smoking, including distance from doorways, non-smoking buildings and even full smoking bans on campus for some universities, these smoking limitations often only serve to protect non-smoking populations from exposure to second-hand smoke," says engineer Michael Waring.

The team filtered the atmosphere in the classroom and sifted out sub-micrometer sized particles.

Roughly a third of the total mass of these materials carried a chemical signature that identified them as third-hand cigarette smoke – compounds produced by burning tobacco that cling to surfaces.

In another test, the team filled a Pyrex container with cigarette smoke before clearing it out.

A day later they passed air from outside through the container again, and found a 13 percent increase in those signature compounds, suggesting whatever was clinging to the sides of the container was still able to become airborne.

It became clear that the third-hand smoke was evaporating into a gas before settling on water particles that were small enough to drift on through the building's internal air currents.

"The fact that third-hand smoke can attach to them, like it would to the clothing or furniture of a smoker, means that the potentially toxic chemicals associated with third-hand smoke are found in places we wouldn't have expected," says atmospheric chemist Peter DeCarlo.

This isn't simply a case of stale cigarette fragrance wafting off a smoker's jacket, either.

This transfer from second hand smoke to surface to tiny water particles points to a chain that can carry the chemicals much further than we first realised.

"What we'd actually uncovered was a new exposure route for third-hand smoke – through aerosol particles, which are ubiquitous in the indoor environment," says DeCarlo.

What's more, those tiny droplets that grab onto the gas molecules fluctuate throughout the year, meaning some seasons might be worse for breathing in third-hand smoke than others.

But be before you panic, it's important to note that it's not entirely clear just how toxic these floating particles are.

A study published in 2017 showed third-hand smoke clinging to fabrics had a significant biological effect on lab mice, but getting enough evidence to demonstrate a solid link between third-hand aerosols and human health in the real world could be tricky.

Meanwhile, we need to decide how to effectively manage potential risks, especially on behalf of those who wish to completely avoid them.

"While most people expect that they'll be exposed to car exhaust, or other chemicals in low concentrations when they're outside – they tend to think that they're escaping all that when they step indoors," says DeCarlo.

"Understanding that we are constantly exposed to these chemicals, even in our workplaces, is a challenge to communicate to the general population."

This research was published in Science Advances.