I'm in Canada and the UV index (which is based on UVB levels) is still hitting 4 or 5 on clear days. That is above the level where you are recommended to wear

I'm in Canada and the UV index (which is based on UVB levels) is still hitting 4 or 5 on clear days. That is above the level where you are recommended to wear clothing, hats, sunscreen, etc to avoid burning. How do you figure that is not enough to produce vitamin D?