From when exactly to use sunscreen to whether beach umbrellas really work, this is what scientific research tells us about staying safe in the Sun.

During the hot, sticky days of summer, sunscreen is an important accessory for most of us when going outside. But there are also a surprising number of myths about it. These are often perpetuated by wellness influencers offering their own sunscreen "alternatives", but some are shared by more authoritative sources, while others seem to have just crept into common acceptance.

So what does the skin protection factor (SPF) really mean in terms of protection? Does ultraviolent A (UVA) alone cause ageing? And do sunscreens really require 20 minutes after applying before they "activate"? Here's what the research and leading experts say we really need to know.

The higher the SPF rating, the more protection provided. But the number indicates a different value than many people think. "SPF does not tell you how long you can be in the Sun without getting sunburn," the US Food and Drug Administration (FDA) notes. "SPF does not mean that if you normally burn in one hour and you apply an SPF eight sunscreen, it will take eight hours for you to burn."

Instead, it's a ratio: the amount of ultraviolet (UV) radiation that would cause the skin to burn with sunscreen, divided by the amount of UV radiation that would cause the skin to burn without sunscreen. This generally works out as the percentage of sunburn-causing radiation that's still transmitted: a sunscreen with a rating of SPF 25, for example, allows in one-twenty-fifth of sunburning rays, or 4%, while SPF 50 allows in one-fiftieth, or 2%. In other words, a SPF 25 blocks 96% of UV radiation and a SPF 50 blocks 98%.

It's important to note that this is calculated by laboratory tests, where sunscreen is applied at an amount much higher (2mg/cm2 of skin) than most people apply, so most of us are getting more UV exposure than these numbers indicate.

Either way, sunscreen needs to be reapplied regularly throughout the day (at least every two hours, as well as after swimming or sweating) for it to work. For the best protection, look for both a high SPF rating as well a sunscreen that offers "broad spectrum" protection. In the UK, the UVA star rating, which indicates a sunscreen's effectiveness against long-wave UVA rays, where five is the highest level of protection.

The spectrum of UV light can be broken down into smaller bands of different wavelengths. UVA has a longer wavelength that can penetrate the skin more deeply, while UVB waves are slightly shorter and only penetrate the outer layers of the skin. However, this doesn't mean that UVB doesn't cause deeper damage.

It was long thought that UVB was the main cause of skin cancer, but in recent decades research has indicated that damage caused by UVA may also lie behind the development of skin cancers. This has led to sunscreen formulations that block both types of UV radiation.

When either form of UV hits the skin, the skin's cells respond by releasing mediators like cytokines – which cause inflammation deeper down, contributing to long-term damage, ageing and cancer risk, says Antony Young, professor emeritus of experimental photobiology at King's College London and a lifelong researcher of sunscreen efficacy.

Both UVB and UVA also contribute to ageing and to tanning, says Mary Sommerlad, a consultant dermatologist based in London and a spokesperson for the British Skin Foundation, a UK health charity. In fact, she says, if she has a patient with a condition like melasma, where there are darker patches of skin, she tells them they need to protect themselves from the broad UV spectrum – UVA included – because UVA will "drive the darkening of the skin", she says.

Many of us skip the hat and sunscreen when we see cloud cover. But that may be a bad idea.

This is because, while clouds do scatter radiation from the Sun, the impact on surface-level UV radiation levels can vary hugely. Very thick clouds can attenuate around 99% of the UV radiation that reaches the Earth's surface, but thinner cloud cover or scattered clouds might have no effect at all. Some types of cloud formations can mean that even more........

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