By Jon Kay
BBC News correspondent
Drinkers may feel they've been breathing more freely in England's pubs since the start of the smoking ban - but has there been any real effect on air quality?
Has the ban made a difference? We employed this monitoring device
Bristol has long connections with nicotine. For centuries, tobacco was brought by ship into the docks here. Then the leaves were rolled into cigarettes and cigars in factories across the city.
Bristol's skyline is dominated by the University's giant Wills Tower - built by the city's most famous tobacco dynasty.
The Cornubia pub is tucked away in a narrow back-street, close to the city centre. A classic British boozer, it's been serving Bristol's drinkers for more then two hundred years. The walls and windows are stained with decades of tobacco smoke exhaled by the after-work crowd who congregate here.
Behind the bar are Ross Nichol and Karen Beesley - a young couple, who took over last autumn. The Cornubia is their first pub. As non-smokers themselves, they were looking forward to the law-change - not just because their hair and clothes used to stink of tobacco but because they were worried about the effects of second-hand smoke.
"You know passive smoking is bad for you," said Ross, "but you tend to block it out of your mind when you work in a pub. You try to forget."
COUNTDOWN TO THE BAN
On 1 July, smoking in enclosed public places was banned across the UK
Northern Ireland, Scotland and Wales already had bans in place
The Magazine counted down the weeks with a series of articles about the impact of the ban on life in Britain (see link below)
"I'd love to know exactly what we've been exposed to," Karen added, smiling nervously.
We decided to show her. As part of our series, we commissioned some scientific research, to measure the air pollution in the pub (and the smoke-related chemicals in their bodies) both before and after the ban.
Two weeks ago - on the eve of England's ban - we took some readings, which were then processed and analysed by the Institute for Occupational Health, who do similar research for the government.
To start with, scientists used an air-particulate monitoring device to measure the small particles from cigarette smoke which can get deep into the lungs.
According to the World Health Organisation, a reading of 65 micrograms per cubic metre of air is considered unhealthy. A reading of 150 is classed as very unhealthy. And 250 is hazardous to health. The reading at the Cornubia was a massive 585 micrograms - that's more than twice the hazardous level.
"Oh my God!" sais Ross, "that's terrifying."
Air pollution expert Professor Jim Longhurst from the University of the West of England says anyone exposed to "those kind of concentrations, day in, day out - there would be an effect on your health. Of that I am sure"
That wasn't all we measured. On the same night, we took Ross and Karen's carbon monoxide levels, by getting them to blow into a special device about half way through their evening shift.
Carbon monoxide is found in cigarette smoke, car exhaust fumes and faulty boilers. High levels can make your blood "sticky". The arteries can become coated and it can lead to high blood pressure.
Before the ban: Inside Bristol's Cornubia
Again, Ross and Karen were alarmed by the readings. Their breath had about seven parts per million of carbon monoxide - which is about the same as you would find in a regular light smoker. And, remember, Ross and Karen thought they were healthy-living and smoke-free!
Finally, we measured the cotinine levels in the their bodies by taking saliva samples. Cotinine is a by-product of nicotine. Their levels were equivalent to smoking around 300 cigarettes each per year.
What's more, their levels were 15 times higher than the average for English non-smokers.
"That's scary" said Karen. "I'm looking forward to the ban even more now."
Fiona Andrews, the South West's Regional Tobacco Policy Manager, says the couple were right to be concerned.
"Studies have proved that you have a 25% increased risk of heart disease through second-hand smoke exposure and similarly with cancer."
The government estimates that about 700 deaths each year are due to workplace exposure to smoke. Ministers hope the bans in Northern Ireland, Scotland, Wales and now England will significantly reduce the number of deaths.
The good news for Ross and Karen is that, since the ban was introduced, things at the Cornubia have already improved dramatically. Comparative readings taken this week show their CO levels are now negligible and their cotinine readings have reverted to the level of a typical non-smoker.
But the most astounding change is in the overall air quality. Pollution levels have dropped from that "hazardous" reading of 585 micrograms per cubic metre - to just 7 micrograms! Gone, almost entirely, in a puff of smoke.