Answer: Automotive Design

There’s a statistically higher rate of skin cancer on the left side of the body in the United States thanks to a simple factor. It’s not because most Americans prefer to keep the sun to their left side by facing north toward their Canadian neighbors in the evening or any such obscure behavior, but simply because Americans drive on the right side of the road and are seated on the left side of the car while doing so.

Although automotive glass does have a mild sunblock-like effect, it doesn’t block all UV light and plenty of people in warmer climates ride around with the windows down and the sun shining directly on their left arm and the left side of their body.

The take away from this is that you can’t trust the window to totally protect you from the sun and even when you’re riding with the breeze keeping your arm cool, you still need sunscreen to protect yourself from the harsh effects of excessive sun exposure.