If you’ve experienced one or more of the following symptoms: rash, fatigue, joint pain, headache or nausea, and it was after a day out in the sun, chances are you suffer from a condition called photosensitivity. You may have a mild form of it, or suffer from severe reactions to sunlight. It may be temporary, from that acne drug you’re taking. Or it may be more permanent because you have an auto-immune condition like lupus. No matter which of these boxes you tick, chances are that you ask yourself the same question —

How much sun is too much?

Your doctor might have already made you aware that the main component of sunlight that affects you is ultraviolet (UV). The reality is that ultraviolet radiation is the most confounding of the sun’s many gifts. None of the human senses are built to perceive it. The eyes don’t see it. The skin doesn’t feel it. Some amount of it is necessary for healthy living. Yet the body suffers reactions when overexposed to it, even in healthy individuals.

The key to coming to terms with UV is the understanding that it’s not momentary UV that affects us. Rather, it’s the accumulation, over time, of UV exposure — a term which, in medicine, is called the UV dose. The UV dose is the product of the UV strength and the time for which UV exposure occurs. This mnemonic summarizes the UV dose:

UV Dose = How Strong (is the UV) x How Long (did you spend in it)

If you guessed it’s riskier to go jogging for 30 minutes, even on a winter morning, then you might have already grasped the concept of the UV dose. Even if the UV was 10 times weaker on the winter morning, you would still end up with 50% more UV dose as compared to the walking situation. The correct answer, however, is that it could be either depending on how strong the UV is. If the UV was 20 times weaker instead (this is not uncommon, especially early on a winter morning), then the short summer stroll would lead to experiencing a higher UV dose.

T o understand what drives this mechanism, without getting into too much medical detail, it’s important to understand that the human body is equipped to repair damage. Ultraviolet radiation is a stressor, because it consists of high-energy photons that are able to penetrate the skin. When exposed to UV, cells undergo DNA damage, but the immune system copes with this and repairs it over time. When the rate of damage exceeds the rate of repair, the damage starts to accumulate. Even in this situation, the immune system can process a certain backlog, but not an infinite amount. The symptoms of photosensitivity occur when the accumulated quantity of damage exceeds the immune system’s processing capabilities.

A simple way to imagine this is to picture being exposed to the stressor (UV) as a source of water (as in the image below). The accumulated water in the reservoir is equivalent to the UV dose. The body’s repair mechanism is akin to a trickle of water out of the reservoir. Once the reservoir gets full, that’s when the body has exhausted its capability for repairing UV damage. This signals the onset of inflammation, which manifests itself in symptoms.

Everyone, whether photosensitive or not, receives the same UV dose when out in the same environment for the same time. But the difference in response to UV lies in the repair mechanism, which is impaired for those who are photosensitive. That trickle of water from the reservoir is more like a slow drip. In extreme cases like xeroderma pigmentosum (XP), there is little to no repair. But for most sufferers of photosensitivity, there is hope. Symptoms are not experienced on incurring just a few moments of UV exposure. The body is capable of tolerating a certain UV dose over the course of a day. And when the UV exposure is removed, it clears up the damage caused by that UV dose over time. Think of the same reservoir as above, but this time the tap is on only briefly. Once the tap is shut off, the reservoir drains out over time, restoring normalcy.

So you can walk to your car and back without suffering adverse effects. You might be able to take that 2 minute stroll in sunshine. You can possibly even undertake that 20 minute jog outside. It all depends on your personal UV dose and how much you can tolerate.

There are two tools in your armory to manage your personal UV dose. The first is using sunscreen. Sunscreen reduces the magnitude of momentary UV exposure you are exposed to, and by consequence, the accumulated UV dose. If applied correctly, it actually reduces the UV exposure by a factor equal to the SPF e.g. an SPF 30 sunscreen would reduce UV exposure by a factor of 30! Sunscreen usage is not without its challenges though — most of us rarely apply the required amount of sunscreen to achieve the rated SPF, and it’s easy to forget to re-apply after the recommended duration.

The second tool for managing your personal UV dose is budgeting. Think of your spending habits. You have a fixed budget, possibly a monthly one. You try to constrain your total monthly expenditure to be within budget. If you exceed your budget, adverse events occur. The same applies to your UV dose. You can “spend” your UV dose budget by excursions outdoors — walking, biking, driving, or just sitting on the park bench — as long as you don’t exceed a budget you set yourself.

So the next time you feel like your lifestyle is constrained by being sensitive to sunlight, remember that what affects you is a combination of how strong the UV is and how long you spend in it. You can trade off one for the other, for example by taking that walk early in the morning when the UV is low. Remember that there are tools to help you in the quest to manage your personal UV dose. Using these, either individually or in combination with each other, lets you go outside worry-free and enjoy the great outdoors — just like you were meant to.

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