Rea Loupa and colleagues, from Democritus University of Thrace, Greece, monitored the air quality in two mediaeval churches in Cyprus, where both incense and candles were burned. Particulate matter and black carbon were measured over six consecutive days, both inside and outside the church of St Paraskevi, near Yeroskipou, and St John's cathedral in Nicosia.

At St Paraskevi candle burning was not allowed, but incense was burned during mass. Meanwhile, St John's allowed candle burning at any time. Loupa and her colleagues found that peak particulate concentrations inside St John's coincided with peaks in candle burnings, and that at these times the concentration of particles in the small size range (0.5 to 1.0μm) were more than 10 times greater than those found outside, despite the fact that there was a busy road next to the cathedral in Nicosia. Meanwhile, black-carbon levels were nearly 12 times higher than outdoors after mass, coinciding with the burning of incense and candles.

Exposure to pollution at these levels is almost certainly not good for human health. "Incense and candles can emit ultrafine, lung-damaging particulate matter that's capable of penetrating deep into the lungs," Loupa told environmentalresearchweb. "From our personal communication with more than a hundred priests they have health problems that possibly are work related, such as asthma aggravation, allergy-like symptoms, irritation of the respiratory tract and lung cancer."

Previously Loupa and her colleagues have shown that on the busiest occasions, such as Easter services, nitrogen oxides exceeded 200 parts per billion, further adding to the toxic soup. "These are the levels you would expect in the centre of Athens, next to a busy road," says Loupa. Elderly people, those with pre-existing respiratory problems, and people who tend to spend long periods of time in churches are probably most at risk.

And it isn't just people who are suffering. The air pollution is also damaging to artwork including the ancient frescos, textiles and rare books often found inside these churches. Nitrogen oxides and ozone cause colours to fade and decrease the strength of materials, while soot deposits directly onto the paintings.

So what can be done to solve the problem? Loupa suggests that the burning of candles and incense should always be carried out in well ventilated areas, both at home and in church, and if possible churches should try to burn candles outside rather than inside. What's more, the quantity of incense burned on each occasion should be as small as possible.

Although incense emits more than 20 times more small particles than candles according to laboratory studies, it is candles that are often responsible for the majority of particles in church air. That's because they are lit by visitors and left to burn all day, whereas incense is usually only lit for a couple of minutes during mass. In some cases, a ban on burning candles inside the church could make a huge difference.

Meanwhile, artwork should be sealed under protective glass if possible or else kept in a sealed environment away from the main sources of pollution.

Author Kate Ravilious is a contributing editor to environmentalresearchweb.