Damaging ultraviolet (UV) rays from the sun are most intense on a clear day when the sun is directly above you. To a lesser extent, UV radiation will penetrate air pollution, clouds, and overcast conditions. Protecting yourself from the sun is always an issue in daytime, unless your trips last only a few minutes or UV levels are very low.
Preventing skin cancer and other health problems is not the only reason to protect your skin from the sun's UV rays. The sun quickly creates tanning or burning on lighter colored skin, which results in a two-tone body color that sometimes looks weird. Worse, over longer time periods, you get dark spots and a leathery texture to your skin; I've read that this is true even for naturally dark skin.
Many would argue that smooth and unblemished skin is the single most important factor in physical attractiveness (or at least a youthful appearance). If you care about your appearance, avoid ultraviolet exposure from the sun.
On the other hand, scientific evidence suggests that sun exposure can actually reduce the risk of certain serious diseases. Some UV rays, when reaching your skin, induce Vitamin D production in your body. It's thought that the Vitamin D helps prevent those diseases. However, in the the U.S. and many other parts of the world, UV levels may be inadequate during part of the year; that's especially true for dark-skinned people or for anyone living far from the equator in winter.
The only other effective way to get Vitamin D is to take supplements, such as cod liver oil in gelatin capsules. Assuming that this is safe and effective, it lets you get Vitamin D all year without the problems of sun exposure.
If you have a flexible schedule, avoid traveling when the sun is high in the sky. One possible rule to use: if your shadow is shorter than you are, it's not a good time to bicycle. You can either travel in the dark or wait until there will be just barely an acceptable amount of light during your whole trip.
There could be an exception to that rule, if you normally cover all of your skin except your face, using clothing like the items described in this page. You might actually prefer to have the sun above you, so that it cannot directly shine on your face.
Many dermatologists recommend high SPF sunscreen lotion for the skin you don't cover.
I don't like the sticky feel of lotion, but I've read reports claiming the non-sticky kind is not effective. It takes time to apply and wash off the lotion. Also, I wonder about what chemicals I'm absorbing through my skin. Anyway, if I commute any kind of significant distance, I feel that I have been somewhat burned anyway.
However, I am unwilling to totally cover myself with cloth or to bike only at night, so I guess that sunscreen is the best alternative.
Different clothes offer surprisingly different amounts of sun protection. Light colored, loosely meshed, minimal-coverage clothing will keep you cooler than full-length clothes with dark and tightly woven fabric. Unfortunately, the cooler kind of clothing is often the least capable of blocking dangerous UV rays from the sun. It's best to seek out clothes specifically designed for sun protection in hot weather. You might be interested in:
Long lycra cycling tights might give a nice combination of sun protection and moisture wicking. But I have never used biker tights; I prefer to wear clothing that looks more like casual wear when I'm off the bike.
One of my favorite pants for sun protection is the RailRiders Eco-Mesh Pant.
The side of each pantleg has two zippers to let you control how much ventilation is possible. You can open most of the legs' length while pedaling, then zip them close when it gets cold. So you could go bike somewhere during the day, come home at night when it's cold, and this pant will keep you comfortable. It does not shield you from rain, though. Closing the zipper hides that white mesh, so the pant looks more like regular clothing; that's nice for when you are off the bike and want a normal appearance.
When the sides are zipped closed, these resemble a normal athletic pant, except for the tan color. Actually, it also looks like a khaki pant, except for the nylon fabric.
I also use nylon hiking pants, with good results. Mine are convertible to shorts using zippers that go around each thigh; if I open those zippers at least halfway, I get ventilation but the pantlegs stay attached. Each pantleg also has a zipper running from ankle to mid-calf that provides more airflow. A bonus is that I can choose to be wearing shorts at my destination, without having to go change clothes.
Unfortunately, the waterproof clothes I feature in this website are no good in sunny weather; even the tear-away pants get too sticky and moist inside. But breatheable cold weather clothing works fine unless it's really sunny and hot.
I use tear-away pants for the extra ventilation created by the gaps between the buttons running down the sides. I don't unsnap the buttons to remove the pant, though I think that is the buttons' main intended purpose. However, if I started to wear shorts underneath, then I suppose I could tear off the pants when I get too hot.
A long-sleeve cycling jersey would give some sun protection combined with moisture wicking.
When wearing a t-shirt, you can put on the special SunGrubbies sun-protecting sleeves shown below.
If you don't like that style, you could get a more form fitting driving sleeve from Coolibar or from Solartex. By slipping either of these under the sleeves of your t-shirt, you will have a style which hopefully will look OK. They also offer a higher level of sun protection, I think because they don't include mesh material.
Another nice option is a specially ventilated long-sleeve shirt. Pictured below is the RailRiders Eco-Mesh Shirt. It's hard to see in the left-hand picture below, but mesh runs all along each sleeve's bottom edge and continues all the way down the shirt's side. The back also has mesh, but that's covered, as seen in the photo on the right below.
An alternative long-sleeve is the SunGrubbies Ex Officio Air Strip Shirt. I've never tried it, but it seems to have similar features.
I've had good luck with a Supplex nylon hiking shirt. It has no visible mesh but breathes just fine, helped by a hidden back mesh that's bigger and better than the RailRiders one pictured above.
Full-finger cycling gloves with a long cuff might provide acceptable coverage. I instead use Sun Precautions' Solumbra sun gloves. Coolibar's full-finger gloves probably work well too.
Ultraviolet rays are harmful to the eyes, so see my page about eye protection.
You can find sun hats with drapes you can secure across your face, as modeled below. I never do this myself, but maybe I should!
Wearing that lets you do two things:
The hat shown above is called Le Hood. I got it from a company that no longer carries it. It's made in Australia, and you'd have to search the web to find a reseller. But some of the company websites I listed above will have similar items.
This might sound silly, but I feel self-conscious about totally covering my face with cloth. Nobody ever does that around here. I use the drapes of that hat to just cover the back of my neck in the legionnaire style. I tuck the fabric under my shirt collar.
If you already own a legionnaire hat, you could combine it with a Coolibar face covering. Or wear a black bandana, which is an occasionally-seen biker style (where I live, at least). This combination would ventilate better than the item pictured above.