Living as we do beneath a giant celestial tap, we might begrudge the health mantra on overexposure to sunshine, but few of us would want to dispute it. Only an estimated half of Britons actually follow the sun-sense edicts, but it is probably safe to say that we know them by rote: cover up, seek the shade, don't brave the midday sun and always, always use a sunscreen. Otherwise, we are told, we face the consequences of prematurely ageing our hides and of increasing, dramatically, the likelihood of getting skin cancer.

Nobody likes wrinkles showing up before they're due, while skin cancer is the most frequently occurring form of cancer in Britain. It doesn't kill as often as other cancers, such as lung and breast, but it nonetheless claims 2,120 lives each year, according to Cancer Research UK. To suggest, therefore, that the sunscreen mantra might be suspect just reeks of irresponsibility.

If increasing numbers of us are getting skin cancer - it has risen threefold in the past 20 years - while too few of us are using sunscreens, you'd think part of the public health mission would be to persuade even more of us to slather on even greater quantities of those sun protection factors (SPFs). But here's the curious thing: skin cancer rates and sunscreen use have both been rising simultaneously.

You could mitigate that in plenty of perfectly reasonable ways. You could say that we don't use the stuff properly, and that we now spend more time on beaches and tennis courts, say, rather than down mine shafts. You could argue that, since skin cancer takes years to show up, it is too early to tell if modern, more protective sunscreens are actually working. Or you could not mitigate at all and say that the raised levels of both sunscreen use and skin cancer is a paradox. And it's into that paradox that all the fears are falling.

Some scientists have long been claiming that the chemicals commonly used in those sunscreens may be more trouble than they're worth. Our use of sunscreen products is more accurately described as a large-scale abuse, since we treat them as a licence to sunbake with abandon.

Nowadays, most sunscreens are "broad-spectrum", meaning that they protect against both ultraviolet-A (UVA) and ultraviolet-B (UVB) rays. The former pass through clouds and glass, are responsible for the sort of skin damage that shows up as premature ageing, and have, in latter years, been linked to sunburn. UVB, meanwhile, are the far stronger rays; though partly blocked by Britain's cloud canopy during winter, UVB light is known as the "killer ray" because it causes sunburn.

The counter-sun products we currently use are either chemical or physical sunscreens, which, confusingly, are still chemicals. Dr Stephen Antczak, co-author of Cosmetics Unmasked, a guide to safe and allergy-free products, explains the distinction: "Physical screens contain reflective materials that reflect the ultraviolet rays away from the skin; chemical sunscreens contain UV absorbers that absorb the ultraviolet rays and reduce their energy," he says.

Even a total science dunce would guess that chemicals capable of changing ultraviolet light are both complex and potent. "They all have gigantic names, some of them going to two or three lines, so that manufacturers have to invent trade names to fit them on to the labels," says Antczak.

Abbreviated, those names are still confounding: para-amino benzoic acid, cinnamates and benzophenones are the three most commonly used in sunscreens. The trouble with these sponge-like chemicals, according to Dr John Knowland, a biochemist at Oxford University, is precisely their mopping-up properties.

"A sunscreen that absorbs energy cannot destroy that energy, it has to do something with it," he says. Knowland, among others, is concerned that UV-absorbing screens react with light and then interfere with the skin, causing DNA damage - one of the possible causes of cancer.

A spokesperson for Garnier, owned by L'Oréal, which makes Ambre Solaire, says, "Sunscreens do not cause DNA damage - it is the UV light from the sun that could cause harm." And Dr Mike Brown, sun-care scientific adviser for Boots, Britain's biggest sunscreen manufacturer, says, "All the evidence I have seen has used non-human DNA that is put in direct contact with sunscreen, then irradiated. That's a totally unrealistic test."

There are, moreover, reports that sunscreen chemicals, rather like the nicotine in stop-smoking patches, can penetrate the skin. In Drop-Dead Gorgeous: Protecting Yourself From The Hidden Dangers Of Cosmetics, author Kim Erickson cites a lab study in Queensland, Australia, where the common sunscreen chemical oxybenzone broke through the skin's protective barrier.

Scientists at Sahlgrenska University in Sweden, meanwhile, found that benzophenone-3 (B-3), another chemical UV-absorber, showed up in the urine of volunteers who had applied the recommended amount of a common sunscreen - in some cases up to 48 hours later.

In Switzerland, researchers testing five common sunscreen chemicals found that they all behaved like the female hormone oestrogen. One of those chemicals tested has been found in human breast milk, suggesting, according to the Women's Environmental Network, that this substance "is absorbed not just into but through our skin, and is accumulating in our fatty tissue".

The Garnier spokesperson says, "We are not aware of any validated scientific research to support these claims."

Brown at Boots, meanwhile, says, "Modern analytic techniques are extremely sensitive at detecting molecules at extremely low levels. Just because they are there, it doesn't mean they are going to be harmful." He adds that the sunscreen ingredients used by Boots fall well below EU guidelines on known no-risk levels.

We don't yet know how worried to be, if at all. The researchers above are calling for caution and more research. Similarly, we don't know if finding these chemicals in fish, animals and water is bad news - although we instinctively recoil at such information.

Meanwhile, there is a similar level of confusion over non-chemical sunscreens, the ones that diffuse, rather than absorb, UV light.

Take titanium oxide, a common physical screen. This potent compound is also used in self-cleaning windows, because it reacts with light to break down organic matter such as dirt. The worry is that it similarly breaks down human DNA, although manufacturers say they use a nicer version of titanium oxide and coat it with silicates to avoid such a possibility.

The European commission's scientific advisory body on cosmetics, meanwhile, advises that titanium oxide, at guideline quantities, is "safe for use in cosmetic products". More recently, this substance has become one of the forerunners of nanotechnology, a process that miniaturises substances so that they are less than a thousandth of the size of a human hair.

The idea is to make titanium oxide particles disappear, since in sunscreens this compound has a habit of sitting visibly and unattractively on the skin. But whether such shrunken particles could more readily enter human cells, is an issue that, according to Knowland at Oxford University, "has not yet been adequately addressed".

Are there natural alternatives to sunscreens? It depends who you ask. The Swedish researchers who tested UV-absorbing sunscreens recommend that we switch to light-deflecting products. The Women's Environmental Network cites Dr Hauschka, Weleda and Green People products as clean, environmentally aware examples of these physical sunscreens.

The US company TerrEssentials, meanwhile, makes a range of natural and organic personal care products - but not sunscreen. One of its founders, Diana Kay, says "We are not saying you should risk photo-ageing, but we haven't found anything that we can recommend as a sunscreen."

There exists a popular misconception that sunscreens protect against skin cancer, although manufacturers never say this. Sunscreens have been shown, in lab rats, to protect against squamous cell cancer, which is one of the three types of skin cancer, occurring more frequently than the potentially deadly melanomas, but far less than basal-cell cancer - the most prevalent and most treatable form. For these two skin cancers, even the World Health Organisation, in an obscure report in 2000, reported insufficient evidence that sunscreen works as a defence.

"No conclusion can be drawn about the cancer-preventive activity of topical use of sunscreens against basal-cell carcinoma and cutaneous melanoma," reported the WHO research panel, which included scientists, medics and sunscreen manufacturers.

The problem is that lab rats get only squamous cell cancer, so it's not possible to test for the other types. Brown at Boots, however, says that sunscreens are still expected to work to help prevent skin cancer. Meanwhile, all manufacturers state that they are not responsible for any misinformation over sunscreen and cancer prevention.

Garnier's spokesperson says that it "does not and never has claimed that sunscreens prevent cancer". It's the same story at Boots: "We do not promote our products with reference to skin cancer," says Brown, adding that the 1939 Cancer Act bans such an association. What the products do say is that they protect against burning UV rays. And, almost universally, dermatologists advise that even a single case of sunburn will increase your skin's susceptibility to skin cancer. Such a link, however, has been questioned.

When Marianne Berwick, head of the epidemiology and cancer prevention unit at the University of New Mexico, reviewed the literature on sunburn and skin cancer, and conducted her own, large-scale population study, she found that, put simply, people couldn't be trusted.

"When we asked the same question at different times, people often gave inconsistent answers about their sunburn history." Berwick concludes that it is not the sunburn in itself, but the exposure to UV light, even while wearing sunscreen, that is more likely to be the risk factor. So why do we believe that sunscreens may protect us from skin cancer? Because it's easier to, says professor of dermatology Gasparro, who argues that the universal advice on this subject assumes we are all going to react to the sun like the mice used in lab tests. But in reality, the only categorically mice-like people are fair-skinned, fair-haired and light-eyed.

"The biggest issue with skin cancer is your genes," he says. "Some people may be able to stay out in the sun for longer than others - but you can't give that out as a public health message." The idea of conveying some kind of simple, cover-all-bases message is, he says, what informs dermatologists' recommendations on sunscreen use. However, in recent years dermatologists and cancer research groups have demoted sunscreen to a third line of defence - for occasions when you absolutely cannot cover up or get out of the sun.

It might sound obvious, but SPF ratings are accurate only if you apply the lotions in the prescribed quantities of 2mg per cm squared - that's about a sixth of a bottle each time you slap it on all over. And who does that? Though we in Britain spend £168m on suncare products per year, that more likely breaks down to a third of a bottle each, per year.

"To simply say 'use more' is a daft thing to do, because people use as much as they feel comfortable with," says Professor Brian Diffey, clinical director of regional medical physics at Newcastle general hospital. He suggests manufacturers should reduce the amount of product used in test cases to reflect what people use on the beach. But Brown says they can't - those SPF quantity ratings are the same the world over and, while the EU is OK with changing them, the US is not.

In any case, the message our confused ears are hearing is that if we wear an SPF of, say, 15, we can loll about in the sun for 15 times as long as we ordinarily could without getting burned. The worry is that we view sunscreen as some kind of invincible shield - even though we don't know for sure that it is any such thing.

You might think that SPF ratings encourage us to see what we can get away with in the sun. However, Brown counters, "We make a lot of effort to say that sunscreens are just a part of your sun defence strategy. We have changed the way we explain SPF and no longer talk about it in terms of the '10 times longer in the sun' approach."

Meanwhile, those SPFs are popping up all over the place, in foundations, moisturisers and lipsticks. Perhaps manufacturers stick the sunscreens into these pots and potions just because they think we like it - and will pay for it. But Antczak suggests a few alternate reasons. First, loading moisturisers with sunscreens corroborates any product claims on wrinkle management, since it is known that these screens offer protection against the kind of premature ageing brought on by exposure to UV light. Meanwhile, many everyday moisturisers are actually skin peelers, which is how they make good the claim of bestowing younger, fresher skin upon the user.

"The outer layer of skin cells is the one that protects you from UV light," Antczak explains. "If you strip those away with skin peelers, the light can penetrate into the skin and cause damage such as premature ageing. SPFs are there to put back the protection that the peelers strip away."

The single point on which there is any consensus, in a field seemingly saturated with conflicting opinion, is this: in the dead of winter, there is little reason to use sun-screen-enriched moisturiser or make-up - and not just because the very idea of being 15 times more resistant to a nonexistent sun is aggravating.

"During winter, we spend so much time indoors, and the amount of UV around is dramatically less, so that there is no need to be using daily SPF moisturisers," says Brown. Of course, the opinion split emerges again from there on. Wearing sunscreen in winter "is not going to do you any harm, but you are wasting your money", claims Brown, while others pronounce it unwise to overburden your face with chemicals about which very little is known with any degree of certainty.

Kay at TerrEssentials eschews the sunscreen-all-year-round approach and instead takes the cover-up cotton shirt, wide-brimmed hat and shade-seeking path to sun protection for precisely this reason: "I would rather take my chance with Mother Nature than expose myself to the chemicals of man."