It’s peak hour. The heat of motorbike and car engines grows unbearable as you wait for the lights to change.

Toxic fumes pump out of exhaust pipes and choke the already humid air.

The traffic lights slowly count down the seconds until the green light appears.

Around you, tuk-tuk and moto drivers, passengers and pedestrians wear plastic surgical masks.

It’s a common sight in most Southeast Asian countries, where the masks are worn for various reasons – often as an attempt to ward off the transmission of infectious disease, or protect against pollution.

But do they work? Experts say the protective effects could be negligible – in fact, the masks could offer a false sense of security.

Epidemic scares like SARS and the H5N1 influenza virus, “bird flu”, have hit the continent hard – H5N1 has claimed eight lives in Cambodia so far this year – and media reports have shown Asian cities awash with cheap plastic surgical masks.

Both scientists and doctors warn that masks can only trap the biggest particles of dust – while it is the smallest particles that are most dangerous.

University student Chenda Prak said she has worn a medical mask when travelling around Phnom Penh since she was a teenager.

While the 25-year-old admitted she has also worn masks for cosmetic reasons – to cover pimples – their primary use is to keep out dust and protect against air pollution, she said.

“The dust does not go into your nose; you can’t feel it. Same with the pollution – you can’t smell it.”

There’s no doubt about it, Phnom Penh is polluted.

A recent study found that air pollution in the capital is projected to rise as development and the city’s population continue to grow.

Much of the pollution comes from factories using outdated and unregulated means of production, private generators used to supplement unreliable power grids and the burning of biomass fuels, according to Professor Stephen Vermette who specialises in geography and development.

Vermette, who authored the 2011 study Mapping of Airborne Particulates: Phnom Penh, Cambodia, said dust particles from the road are becoming less significant, compared with smaller particles and gasses associated with car exhaust.

The two-stoke engines used in motorbikes produce more emissions, while second-hand vehicles pollute more than newer counterparts, he added.

The pollutants exist in the air as both fine and coarse particles.

“The fine particles are capable of penetrating deep into the lungs, while coarse particles reach the nose and throat.”

He said numerous epidemiological studies have linked the inhalation of fine airborne particles to respiratory and cardiovascular mortality.

“Simply put, the standard of face masks I see worn in Phnom Penh are of little value to protect the wearer, other than, perhaps, from the coarsest of dust particles.”

Tiny particles can pass through and cause lung disease, said Dr Gavin Scott, who runs the Tropical and Travellers Medical Clinic.

“Surgical masks were designed to stop transmission of disease or dirt from the nose or mouth of the surgeon to the exposed insides of a patient.

“They only trap the large dirt particles and the large mucus droplets containing disease, essentially performing the same function as the nasal passages.

“Previously, we thought transmission was largely by large droplets containing the flu virus, which would be trapped by a mask. However, we now know that when someone coughs, the influenza virus can travel by itself [not on mucus droplets] up to six feet away and through a mask.”

When Dr Allison Aiella from Columbia University, who specialises in epidemiology and infectious diseases, conducted a study on over a thousand students in different university halls of residence for influenza, she found that the masks had little influence.

For the study, a third of the students wore surgical masks and continuously washed their hands during the flu season.

Another third just wore a surgical mask and the last third were used as a control and used neither precaution.

There was a significant reduction in the rate of influenza among those participants who wore the facemask and used hand-hygiene intervention.

It cut the number of people contracting influenza in half, she said.

But she added that there were no substantial reductions in influenza in the facemask-only group compared with those who used no precautions.

“There were some reductions, but it was not statistically significant.”

If another 1918 flu epidemic hit us, Dr Scott said, the masks would be useless.

“Best advice would be to stay at least six feet from everyone and wash your hands frequently.”