For many years we have been told a simple public health message:
“Wear sunscreen, especially during the summer.”
It is well-intentioned advice, but as a physicist I have always found it rather unsatisfying. Nature is rarely that simple, or linear (In fact, in this universe, ‘linear’ is the exception rather than the rule!).
If we are trying to reduce tissue damage caused by ultraviolet radiation, surely the important quantity is not the month shown on the calendar, but the amount of ultraviolet energy actually reaching our skin.
This way of thinking has become central to much of my work on medical lasers. Whether we are discussing laser hair removal, tattoo removal, IPL or radiofrequency devices, the underlying physics is always the same:
Energy flows from a source, through a medium, into a biological target – then something happens!!
The biological response depends on the amount of energy that finally reaches that target. Why should sunlight be any different? The Earth is furthest from the Sun during the northern hemisphere summer.
Many people are surprised to learn that the Earth reaches its greatest distance (aphelion) from the Sun in early July. If distance alone determined ultraviolet intensity, we should actually receive less UV during the northern summer than during the winter.
The difference is small—only around 6–7%—because solar intensity follows the familiar inverse-square law (familiar, if you’re a physicist!!). Clearly something much larger must be happening.
It is the solar angle above the horizon that dominates – the dominant factor is not the distance from the Sun, but the geometry. During summer the Sun climbs much higher into the sky. The sunlight therefore:
- strikes the Earth’s surface more perpendicularly,
- travels through less atmosphere before reaching the ground,
- experiences less absorption by ozone,
- undergoes less Rayleigh scattering (which is enormous in the atmosphere!).
Consequently, far more biologically effective ultraviolet radiation reaches the ground when the Sun is ‘high’. Only around 5% of the UVA that reaches the Earth finally reaches the ground. The ultraviolet at the ground level is mostly UVB – which can be very harmful to biological tissues in terms of photo-ageing and potential damage to DNA and RNA.
This is why high-latitude countries like Scotland experience much higher UV levels in July than in January, despite the Earth being slightly further from the Sun.
The UV Index already contains the complicated physics
The UV Index is often misunderstood. It is not simply a measurement of ultraviolet power. Instead, it represents the erythemally-weighted ultraviolet irradiance—in other words, the amount of UV radiation capable of producing ‘sunburn’.
The local UV Index already incorporates:
- latitude,
- season,
- time of day,
- atmospheric ozone,
- cloud cover,
- solar elevation,
- and even the Earth’s changing distance from the Sun.
Rather than calculating all of these individually, we can simply use the measured UV Index – which you can find in all good weather Apps.
Thinking in terms of dose
Physicists usually think in terms of energy and dose rather than labels. A simple first-order relationship is
Skin Dose = UV Dose Rate × Exposure Time
Since one UV Index unit corresponds to an erythemally-weighted irradiance of approximately 25 mW/m², the accumulated dose can be estimated as:
UV Dose ≈ 1.5 × UV Index × Exposure Time
This simple equation immediately explains why there can never be a universal recommendation for sunscreen, because the UV Index is also important! The ‘SPF’ and ‘UVI’ must go hand-in-hand.
A person spending twenty minutes outdoors in Scotland during winter receives a vastly different ultraviolet dose from someone spending twenty minutes on a Mediterranean beach in midsummer.
The calendar alone tells us very little!
So where does sunscreen fit into all this?
Sunscreen does not remove ultraviolet radiation. It is an ‘attenuator’ – it simply reduces the amount reaching the skin, mostly through absorption, and a little through scattering/reflection.
Likewise:
- clothing reduces exposure,
- shade reduces exposure,
- clouds usually reduce exposure (though not always!!),
- shorter exposure times reduce exposure.
These are all simply different methods of reducing the biologically effective energy arriving at the skin. From an engineering perspective, sunscreen is just another attenuator within the overall energy-flow pathway.
The same physics as medical lasers
This way of thinking may sound familiar to anyone who has attended one of my laser training courses. When discussing laser treatments we do not ask:
“Should every patient receive 40 J/cm²?”
Instead I ask:
“How much energy must actually reach the biological target to produce the desired response?”
Exactly the same philosophy applies to sunlight. The important quantity is not whether it happens to be summer. The important quantity is the biologically effective ultraviolet dose received by the skin.
A better public health message…
Perhaps we should replace the traditional advice with something more scientifically accurate:
Protect yourself whenever your expected biologically effective UV dose becomes significant for your skin type (Fitzpatrick) and the time you intend to spend outdoors.
That recommendation automatically accounts for latitude, season, time of day, cloud cover, altitude, ozone and exposure time. In other words, it is based on physics rather than the calendar.
The bigger picture – Fizzics!
Over the past few years I have become increasingly convinced that many optical and thermal treatments can be understood far more clearly by following the flow of energy. Whether we are discussing lasers, intense pulsed light, radiofrequency devices or simply sunlight, the same principle keeps appearing:
Energy leaves the source – some is lost – some is absorbed – only the absorbed, biologically effective energy determines the response!
Perhaps that is the simplest lesson of all. Physics does not care whether the source is a medical laser or the Sun. It only cares where the energy goes. The biology then reacts as it must, and that reaction depends on the amount of absorbed energy and the rate at which it is absorbed (wavelength, fluence and pulsewidth).
I’m currently writing a paper for publication later this year based on this stuff with one or two more equations because I know how you all love equations!!
In the meantime, stay frosty, and SPF’d, people,
Mike.
PS We discussed this in a recent podcast. You can find them here.
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