The last bits of tinsel are down and the fairy lights are back in hibernation. With the dark, cold nights here for a while, there is no better time to light up those scented candles you were given at Christmas. But is it without risk? What chemicals are used to fragrance products we use, and what happens to them once released into the air inside our homes?

Only a few studies have been done to answer that question, yet there is an increasing evidence of “indoor pollution”. Could our air-tight, insulated, ultra-clean homes actually be harming our health? And it’s not only scented candles that we need to consider – in the UK, we are keen users of air fresheners, plug-ins, wipes and everyday cleaning products.

The characteristic “lemon-fresh” or pine scent so familiar in bleaches and washing up liquids comes from fragranced chemicals: limonene that smells like citrus, and alpha pinene that smells like – yes, you guessed it – pine. Both are commonly used. The chemicals themselves are not known to be harmful – it’s what they turn into once released into the air that might concern us.

One of the known secondary products of all fragrance chemicals once they react in the air is formaledehyde, with its carcinogenic and skin and breathing-irritant properties. Everyday exposure to indoor chemicals such as formaldehyde may contribute to increasing incidences of asthma, cancers and other illnesses, and many countries have taken steps to control exposure. In the UK, the levels of formaldehyde in the air and water are strictly regulated by law, and the EU also has rules on the maximum levels of formaldehyde and formaldehyde releasers in cosmetic products.

Formaldehyde belongs to the family of volatile organic compounds (VOCs), a large, diverse class of compounds that are ubiquitous in indoor air. In small concentrations, they are a normal part of our environment. However, exposure to high levels of VOCs indoors is a source of concern among health professionals, particularly their effect on the delicate airways of children. With our homes better insulated than ever, we have compounded the problem – our airtight homes keep inside air, well, inside. This is one area where old, draughty houses are better for us; a good airflow leads to fewer toxic chemicals.

As part of the research for BBC’s Trust Me I’m A Doctor, I went to meet Alastair Lewis, professor of atmospheric chemistry at the University of York. He set up a small experiment with his team to measure levels of chemicals released from household products. The experiment recruited six homes in York, all of them modern houses with a similar outdoor air quality. Lewis placed air samplers in each house for five days to see which homes had the highest chemicals levels, and what those chemicals were.

Three homes were tested for formaldehyde. The lab studies showed that for every two molecules of limonene released, one molecule of formaldehyde was formed. At the start of the experiment, all the houses had similar levels of most chemicals – except the ones from fragrance. Three of the six households had higher levels of limonene, which corresponded to the amount of household products and scented candles used. In fact, one of the houses showed readings higher than Lewis had seen before: four dog walks a day across muddy fields resulted in four daily floor cleans, and constant use of air fresheners to keep the smell of “wet dog” away. Significantly, the experiment found that levels of formaldehyde correlated with the levels of limonene.

So should you throw out the scented candles and let the floors get muddy? Not quite yet. In the 1980s, Nasa published a paper supporting evidence for a simple way of to reduce chemical levels in enclosed space stations. The great scientific breakthrough? The humble houseplant. Certain common plants were shown to remove toxic agents naturally from the air. So, as part of the BBC experiment, we looked at what happens in our earthbound homes when we introduce a simple plant.

Four houseplants were placed in each of the three volunteer families’ houses for four weeks. Lewis again took air samples at the end of the experiment to see whether the plants had changed anything in the levels of chemicals in the air. The plants were chosen based on previous research – chlorophytum (spider plant), dracaena (dragon tree), scindapsus (golden pothos) and Hedera helix (English ivy).

As the nights drew in, and the weather got colder, behaviour within the homes changed accordingly – shutting doors and windows and lighting candles for longer. The final results showed that the limonene increased through this period – probably because of those extra candles and fewer open windows. However, levels of formaldehyde fell in all three homes. The results of this, and other experiments conducted elsewhere, suggests that plants could indeed have played a role in reducing the levels of formaldehyde in the homes.

The key finding, though, is that we can control the levels of these chemicals in our homes by using fewer fragranced products such as air fresheners and scented candles, and choosing fragrance-free cleaning products. And next time you light a candle, open a window as well – and consider a trip to the garden centre.

Saleyha Ahsan is a doctor in the A&E department of a London hospital and one of the presenters of BBC2’s Trust Me, I’m a Doctor.