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KATE Grenville realised in her early 30s that wearing perfume gave her a headache. She could manage that. Then it was other people鈥檚 perfumes too. But things really got out of hand on a recent trip when she was forced to ask her taxi driver to remove the air freshener in his cab, and later caught herself sealing her hotel room door with tape to keep out the smell from corridor fragrance diffusers.

鈥淚 had a nasty feeling that I鈥檇 just crossed one of life鈥檚 little boundaries,鈥� she writes in her new book . 鈥淚t was possible I鈥檇 joined the section of humanity that thinks the moon landings were faked by the CIA.鈥�

In fact, when Grenville started investigating, she found she was far from belonging to such a clique. Surveys suggest that many of us feel negative health effects from fragrances, and if recent headlines are to be believed, our love of a good spritz could be causing asthma, migraines and even cancer. The issue is causing such a stink that some compare it to passive smoking, and are calling for scent-free workplaces and schools.

鈥淭he results are stunning and consistent. In Australia, a third of the population, and in America, over a third of the population, report one or more types of health problems when exposed to fragranced consumer products,鈥� says of the University of Melbourne.

So how worried should we be? Are scented products making people sick, and what should we do about it?

Fragranced goods haven鈥檛 always been so ubiquitous. Prized by the ancient Egyptians and Romans, fragrances started off as luxuries, made from plant extracts and, in the case of musk, animal gland secretions. But these natural products were expensive and hard to come by. From the late 19th century, advances in industrial chemistry meant that synthetic scents could be developed that were much cheaper and more varied. The fragrance industry boomed.

The final step was a marketing push that saw perfumes move from personal care to everyday household products. Advertisers told us things were only truly clean if they also smelled good 鈥� our kitchen surfaces, toilets and laundry all gradually came to smell of lemon and pine. Humans have never been as exposed to synthetic fragrance chemicals as they are today.

Secret scents

This raises questions around what chemicals go into scented products and whether they are safe. But it is hard to know what鈥檚 in these goods. Global fragrance is big business and the ingredients are trade secrets. , and , scent ingredients don鈥檛 have to be declared 鈥� they can instead just be labelled as a 鈥渇ragrance鈥� or 鈥減arfum鈥�. Rules are stricter in the European Union, where the most likely to cause skin reactions must be named.

The onus, then, is on manufacturers to ensure their scented products are safe, rather than requiring every ingredient to be approved by a regulatory authority. One way the industry seeks to do this is through the Research Institute for Fragrance Materials, a US-based body funded largely by companies involved with fragrance or scented products. The RIFM researches the safety of fragrance ingredients and its findings are reviewed by an independent panel of experts. But that doesn鈥檛 solve the problem. 鈥淣o law requires disclosure of all chemicals in a fragranced consumer product,鈥� Steinemann says, 鈥渁nd even if we did have a list of all the chemicals, how would a consumer assess safety?鈥�

The issue is further complicated because the chemicals used readily react with ozone in the air to create new, secondary compounds, and that affects our indoor environments.

鈥淚 notice when I go to the supermarket almost everything smells now. You can even buy scented toilet paper. It鈥檚 bizarre,鈥� says , who studies indoor air quality at the University of Texas at Austin. 鈥淭here certainly has been a big increase and that鈥檚 transformed the nature of buildings. That鈥檚 made buildings more chemically reactive.鈥�

鈥淭he message has been twisted into 鈥榮cented candles give you cancer鈥欌��

If ozone in the air mixes with these scenting agents, 鈥測ou get bursts of literally hundreds of chemical products that come out of the reactions鈥�, says Corsi. 鈥淭here鈥檚 a huge question mark as to whether the products of the reactions 鈥� what they are transformed into 鈥� are harmful to us,鈥� he says.

This reactivity is the basis of many cancer claims made about scented products. The key concern is that when they react with ozone, some of the chemicals in fragrances produce formaldehyde. At high exposures formaldehyde is and can also cause .

Last year, one piece of research was the focus of numerous articles making the cancer claim. The study, carried out by the BBC in collaboration with , who studies atmospheric chemistry at the University of York, UK, found of limonene, which is responsible for citrus scent, in a number of homes. Limonene is one chemical that reacts with ozone to create formaldehyde.

But the media overhyped the findings, says Lewis. 鈥淲e raised this as something worthy of further study,鈥� he says. 鈥淥f course that message has been twisted into 鈥榮cented products give you cancer鈥�, which is not what we were saying at all.鈥�

Corsi agrees that more needs to be done to understand what reactions are taking place in our homes as a result of perfumed products and how this impacts on our health, but says we shouldn鈥檛 be overly worried about formaldehyde. 鈥淕enerally, the amount of formaldehyde that鈥檚 formed is pretty tiny compared to other sources of formaldehyde in the house,鈥� he says.

Formaldehyde in buildings tends to come from wood and furnishings, agrees , at the National Research Centre for the Working Environment in Copenhagen, Denmark. And most Western homes and offices fall well under the limit set by the WHO, he says.

New, are underway to properly probe the chemistry of our indoor environments, but until those results are in, it seems the most far-fetched headlines are just that.

鈥淭he majority of fragrance molecules are present in such low concentrations that, apart from things like sensitisation and migraine headaches, the likelihood of them causing cancer or endocrine disruption is very low,鈥� says , a toxicologist and pharmacologist at the University of Adelaide, Australia.

But that doesn鈥檛 mean there aren鈥檛 other effects. Steinemann has conducted surveys of the health effects of exposure to perfumed products. In one study of more than 1000 , over a third of respondents reported problems, including migraines, skin reactions and respiratory difficulties such as asthma attacks when exposed to fragranced products. Steinemann has found similar reported health impacts in Australia, and expects to publish a UK study this year with comparable findings.

One problem with Steinemann鈥檚 studies is that they are self-reported. 鈥淗er work involves talking to people in short interviews, and just by raising the issue of adverse effects, you are negatively biased,鈥� says Wolkoff.

There may still be cause for concern , however. 鈥淓ven if we don鈥檛 have good, robust experiments with large numbers of people, it certainly seems that some fraction of the population is sensitive to breathing these chemicals,鈥� says Corsi.

One possible cause is the sense of chemesthesis, which is responsible for the tingly, hot sensation you get from spicy food. Most food flavours are transmitted to the brain as smells, but this spicy sensation triggered by chilli peppers, garlic and wasabi instead acts on the brain via the trigeminal nerve.

High concentrations of fragrance chemicals in the air can trigger this irritant system in the nose, throat and airways. 鈥淎s far as we know, any odour can also be an irritant at sufficient concentration, so just about anything people can smell could in theory irritate,鈥� says Paul Wise at the Monell Chemical Senses Center in Philadelphia, Pennsylvania. The sensation produced 鈥渃an be anything from a low-level kind of warming or prickling sensation all the way up to something stinging and frankly painful,鈥� he says.

For most people the concentrations in scents would be too low to irritate, but some people may be more sensitive, says Wise, although little is known about the range of irritation thresholds in the normal population.

As well as general irritation, chemesthesis may also be involved in reported cases of smell-induced migraine. Depending on the country, between 10 and 14 per cent of Steinemann鈥檚 respondents reported migraine headaches after exposure to scents. Some people who experience migraines say perfumes are a trigger and many find smells unpleasant during an attack, a phenomenon called osmophobia. A study of those who get migraines found that .

It鈥檚 also possible to trigger migraines using smell. In another study, people susceptible to migraine were exposed to a floral scent for a minute. More than within an average of 2 hours and 7 per cent reported no headache, but did feel nauseous, which is a migraine symptom.

One theory is that people who get migraines may have normal sensitivity to smell, but an abnormally sensitive sense of chemesthesis. The trigeminal nerve is known to play a role .

But there鈥檚 another explanation, according to Peter Goadsby, a neurologist at King鈥檚 College London who specialises in migraine. He points out that osmophobia can be present in the premonitory phase that comes hours before a headache, a phase many people don鈥檛 even notice.

This poses a problem when it comes to assessing the effect of perfumes, he says. 鈥淭hey could be a chemical trigger 鈥� or it could be that if you are sensitive to smells you notice a perfume in the premonitory phase, and then when 3 hours later you get a headache you think the smell must have triggered the attack, whereas it was going to happen anyway,鈥� he says.

When it comes to asthma, the findings are also mixed. About 8 per cent of Steinemann鈥檚 respondents said fragrances triggered their asthma, and other research has found that cite fragrances as a trigger. But few experiments have examined possible links between fragrance and asthma, and those that have don鈥檛 show an effect.

鈥淭here is no experimental evidence to support [the idea] that people get asthma or exacerbated asthma from exposure to fragrances,鈥� says Wolkoff. He suspects psychology is at work instead.

A big problem with experiments that expose people to fragrances is that blinding is virtually impossible: the subject knows they are smelling a scent and this may affect their response.

But there are ways around this. , a psychologist at Monell Chemical Senses Center in Pennsylvania, who studies psychological aspects of odour perception, recruited 17 people with moderate asthma. She told around half that they would be exposed to a fragrance that could make their condition worse. The rest were told they would smell a scent that could be therapeutic. Both groups were then exposed to the same rose scent.

She expected those who were misled into thinking the scent could do harm to report more symptoms, which indeed they did. 鈥淲hat we didn鈥檛 expect was to see a true physiological change as we did with their exhaled nitric oxide levels, which is a measure of inflammation,鈥� says Dalton. Even 24 hours after exposure to the smells, levels of nitric oxide remained high. Dalton thinks the fear of the scent led to a physiological response and says that stress is known to exacerbate asthma.

These findings suggest it might not matter whether fragrances and the chemicals that make them actually do trigger effects like migraine and asthma 鈥� the consequences are none the less very real for those who experience them. Combine that with the question marks over the chemicals created by reactions between scented products and indoor air, and the fact that our noses are incredibly sensitive, and it鈥檚 understandable that some might want to take action.

鈥淭he fear of a scent alone can lead to a physiological response鈥�

Increasing ventilation and using fragrances in moderation are sensible steps. A more radical solution is to go scent-free, an idea that is gaining ground.

In 2010, the city of Detroit was ordered to make all its workplaces fragrance-free and pay $100,000 to one of its employees who suffered from a chemical sensitivity to scented products, which the court ruled was a disability. The US has a fragrance-free policy in all its offices.

This may not prove popular with all employees, but it can be effective. Corsi has been researching air pollution in Texan high schools, including some with a fragrance-free policy. 鈥淲e do find that the schools that have aggressive no scent policies have much lower levels of these chemicals,鈥� he says.

So if you love scented sprays and feel that your essential oils really are just that, remember not everyone feels the same way 鈥� there might well be someone in the hotel room next to yours taping up the cracks under the door and wondering whether they鈥檝e lost their senses. If someone tells you they have a problem, it鈥檚 not to be sniffed at.

This article appeared in print under the headline 鈥淪omething in the air鈥�