Air inside churches may be a bigger health risk than that beside major roads, research suggests.
Candles can generate pollutants
Church air was found to be considerably higher in carcinogenic polycyclic hydrocarbons than air beside roads travelled by 45,000 vehicles daily.
It also had levels of tiny solid pollutants (PM10s) up to 20 times the European limits.
The study, by Maastricht University, The Netherlands, is published in the European Respiratory Journal.
The researchers say that December, with churches lighting up candles for Christmas, could be an especially dangerous month for the lungs.
It is now believed that respiratory health is increasingly at risk from so-called "indoor pollution" in the home, workplace and other enclosed spaces.
The Dutch team set out to examine the air quality in churches, as they are often poorly ventilated, with candles burning all day, and frequent use of incense. Both could, in principle, be expected to have some harmful effects.
The researchers analysed the particulate matter concentration found in the air of a small chapel and a large basilica in Maastricht following lengthy use of candles or a simulated service in which incense was burned.
Fine particulate matter is a major ingredient in air pollution. Consisting of solid particles with a diameter of 10 microns or less, it contains different types of toxic chemicals, including soot, metals and various carcinogenic molecules.
The particles can penetrate very deep into the lungs and trigger various lung and heart conditions.
The researchers found that, after nine hours of candle-burning, the church air had PM10 levels of 600 to 1000 micrograms per cubic metre - more than four times higher than before the start of the first morning mass.
This represents 12 to 20 times the European allowed average concentration over 24 hours.
The study also found very high concentrations of polycyclic aromatic hydrocarbons, also known to be carcinogenic.
New free radicals
It also uncovered various types of free radicals, including some previously undocumented ones.
Free radicals are highly reactive molecules that damage lung tissue and can trigger or exacerbate inflammatory reactions, including those connected with major respiratory conditions such as asthma and chronic bronchitis.
The researchers say priests and people working for long periods in churches are at greater risk than ordinary worshippers.
However, worshippers devout enough to spend several hours each day in church could also be affected.
Researcher Dr Theo de Kok said: "While we still have to assess more precisely what level of risk these people are running and how toxic the newly identified free radicals are, this discovery is very worrying."
A spokesman for the Church of England said that during candle and incense-burning ceremonies the doors of churches were often open, with people coming in and out.
He said many factors would govern pollution levels, such as the height of the church, and whether the high level windows were open.
He also took issue with the idea that churches were poorly ventilated - pointing out that many are notorious for being draughty.
However, he added: "This study certainly bears further investigation, and we will keep a watching brief."
Dr Richard Russell of the British Thoracic Society, said: "Particle pollution, whether it be in an outdoor or indoor environment, can be a danger to lung health and cause respiratory diseases such as emphesyma and bronchitis.
"More research needs to be done in this area but we would also recommend that churches look at ways to reduce indoor air pollution such as improving ventilation."