Lighting a candle in church may bring you inner peace, but it does not do the paintings much good.
New research shows churchgoers can also damage valuable church artworks by burning incense, polishing the brass and even just breathing.
Frescos, icons, textiles, books and silver are being seriously degraded by poor environmental conditions inside churches according to the study.
It was done by a team from Democritus University of Thrace in Xanthi, Greece.
Rea Loupa and her colleagues monitored air quality in two churches in Cyprus.
Over one summer and one winter season, they recorded variables such as temperature, humidity, light and particulate levels, along with carbon dioxide, ozone and nitrogen dioxide levels.
In addition, they kept a detailed diary at each church, recording all the comings and goings.
Candles and cars
Weather conditions had a strong impact on indoor pollution, but surprisingly church visitors also played a significant role.
"In St John's cathedral in Nicosia, one of the major sources of pollution was burning candles," explains Loupa.
The other church, St Paraskevi near Yeroskipou, didn't allow candles, but still had poor air quality at certain times. "Sometimes this was locally generated by traffic around the church," says Loupa.
On the worst occasions ozone levels inside St John's cathedral reached 40ppb (parts per billion) and nitrogen dioxide 200ppb. "These are the levels you would expect in the centre of Athens, next to a busy road," says Loupa.
International recommendations (from the National Materials Advisory Board) suggest that artworks should not be exposed to more than 1ppb of ozone and 2.5ppb of nitrogen dioxide.
Over time, the pollutants will start to eat into artwork
Both nitrogen oxides and ozone are bad news for artwork because they cause colours to fade and decrease the strength of materials.
They also react with other pollutants and often create secondary products that can be even more harmful.
Hold your breath
Volatile organic compounds such as isoprene were also a problem inside the churches.
"People emit organic compounds in their breath, so if the church is busy this is a problem," says Loupa. But even in quiet churches, organic compounds can be high, with cleaning products such as furniture polish and bleach contributing to the chemical cocktail.
Loupa and her colleagues think that these church pollution problems are likely to be global.
"All over the world there are lots of cars now, creating high levels of nitrogen oxides, ozone and particulate matter," says Loupa. And most churches are cleaned with solvent cleaners, rather than the traditional lemon and vinegar.
The research was conducted in two Cypriot churches
But not everyone is convinced that candles, cars and cleaning products are the worst culprits.
"In the UK, bat urine is one of our biggest problems," says Sally Woodcock, an art conservator who specialises in church paintings. Splashes of bat urine etch through the varnish and bleach the underlying paint.
And the UK's damp climate and dwindling church population is also taking its toll. "When churches are not open much then there is not much air flow. It is the ideal environment for mould," explains Woodcock.
Whether it is bat urine or candle smoke, Loupa and her colleagues think that there are ways of improving church interiors and preserving artwork.
"We suggest that no candles are allowed and that very precious artwork is put under glass," says Loupa. And if possible the scientists also recommend regulating the temperature and humidity with air conditioning and heating.
The research is being published in the journal Atmospheric Environment.