People who smoke cigarettes are pumping out 10 times more toxic air than cars, say experts.
Cigarettes polluted more than cars
Tobacco smoke produced far more fine particulate matter - the element of air pollution most dangerous for health - than diesel exhaust.
The National Cancer Institute team told Tobacco Control the findings could explain why non-smokers exposed to passive smoke get lung damage.
They also support growing calls for a smoking ban in enclosed public spaces.
Particulate matter is known to increase the risk of lung cancer and asthma and in Europe the legal limit of emissions is set at 40ug/m3 per year.
They come from a variety of sources such as cars, trucks, buses, factories and construction sites.
Environmental tobacco smoke is another known cause, which Dr Giovanni Invernizzi and colleagues at the Tobacco Control Unit in Milan said has received less attention.
They conducted an experiment to compare the emission from cigarettes with those from a diesel car.
The controlled experiment was carried out in a private garage in a small mountain town in northern Italy which had very low levels of particulate matter air pollution.
A turbo diesel 2 litre engine, fuelled with low sulphur fuel, was started and left idling for 30 minutes in the garage, with the doors closed, after which the doors were left open for four hours.
Three filter cigarettes were then lit up sequentially, and left smouldering for a further 30 minutes.
A portable analyser took readings every two minutes during the experiments.
Combined particulate levels in the first hour after the engine had been started measured 88 ug/m3.
In comparison, those recorded in the first hour after the cigarettes had been lit measured 830 ug/m3 - 10 times greater.
The diesel engine exhaust doubled the background particulate matter levels found outdoors at its peak.
The environmental tobacco smoke particulate matter reached levels 15 times those measured outdoors.
The authors said: "The present data give cause for concern."
Similarly, researchers from Lund University in Sweden found toxic substances in the air of a smoky room were 120 times higher than in a smoke-free room.
Their findings appear in the journal Indoor Air.
Amanda Sandford from Action on Smoking and Health said: "This research should lead to a greater understanding of how tobacco smoke can trigger respiratory diseases such as asthma.
"While there are many sources of pollution that we don't have much control over, we can control tobacco smoke emissions.
"We already know that smoking is a trigger for asthma. There is no need to wait for more evidence.
"This study underlines the need for a ban on smoking in all indoor public places and workplaces. Every day of delay means that more people are becoming ill through second-hand smoke. The time for action is now."
A spokesman from the Tobacco Manufacturers' Association said: "A garage in the mountains of northern Italy hardly constitutes the controlled laboratory conditions normally required for experiments of this kind.
"Public policy on smoking should be based on sound science."
He said while tobacco smoke could be an "annoyance" to some people, "anti-tobacco bias should not be allowed to distort scientific objectivity."
"The best way of addressing public concerns about environmental tobacco smoke is through the provision of designated non-smoking and smoking areas with good ventilation," he said.