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Ultraviolet A (UVA) – Considered the unhealthy wavelength because it penetrates your skin more deeply and cause more free radical damage. Sunblocks containing SPF filter out the beneficial UVB, not these cancer-causing UVAs, unless they also contain a UVA blocking ingredient.
As a result, wearing sunscreen may prevent you from burning, as excessive UVBs are the chief cause of sunburn, but you still receive a large amount of skin-damaging radiation. Moreover, UVA rays are constantly available, even on cloudy days. There are likely some benefits to UVA in moderation that we do not fully understand, as there appears to be with many spectrums emitted from the sun.

Ultraviolet B (UV – This is the 'healthy' wavelength that helps your skin produce vitamin D. While both UVA and UVB can cause tanning and burning, UVB does so far more rapidly.
Contrary to UVAs, which are more readily available, UVB rays are low in morning and evening, and high at midday or solar noon, making this the most optimal time for vitamin D production (roughly between 10:00 a.m. and 2:00 p.m.). Ironically, this is the timeframe most mainstream experts warn you to stay out of the sun.

As a general rule of thumb, to optimize your vitamin D levels, you need to expose large portions of your skin to the sun – including your legs, back, arms, and chest. For optimal benefit, strive to have at least 40 percent of your skin uncovered.

The key is knowing when to cover back up. You want your skin to turn the lightest shade of pink. This can occur in as little as 10-20 minutes, depending on your skin tone and other factors, such as location and cloud cover.

At that point, you've reached your skin's equilibrium or saturation point, and your body will not produce any more vitamin D. It can take three to six times longer for darkly pigmented skin to reach the equilibrium concentration of skin vitamin D.

You can create as much as 20,000 units of vitamin D per day this way. Best of all, your body has this built-in feedback loop that prevents you from overdosing on the nutrient when you get it via sun exposure.

If you're taking supplemental vitamin D3, not only do you need to get your level tested regularly to ensure you're taking enough to maintain optimal levels (between 50 and 70 ng/ml), you also need to be mindful of increasing your intake of vitamin K2. When using the sun, having and maintaining a tan can serve as a "visual measure" of your vitamin D status.