The answer to your question is that : nobody knows for certain. Our knowledge is not yet comprehensive enough. Each person responds differently,skin pigmentation plays a role etc etc.
It is true that UV light , in excess , can be a factor in the development of skin cancer. But it is also true that UV light is beneficial to the human body. UVB will generate vitamin D in your skin, which has anti-cancer properties. UVA light (longer wavelength) will produce other substances including beta-endorphins, a known antidepressant/painkiller as well as nitric oxide a compound known to be beneficial in the cardiovascular system,it lowers blood pressure.
People that have long term , regular, exposure to sunlight, such as an outdoor worker have less chance of developing melanoma , than say an indoor worker. These outdoor people have higher levels of protective vitamin D circulating in their blood. 95% of skin cancer deaths are attributable to melanoma. The other 5% are attributable to squamous and basal cell carcinomas. These latter types are more frequent in overexposed skin. Melanomas can occur on the least exposed skin , i.e sole of foot, nail bed etc. From this , you should see that it is not a simple equation , UVB or UVA is good or bad.
The answer is that your body requires vitamin D, if you lack it , your body does not have the full complement of tools to fight illnesses of many types, including cancers, autoimmune diseases, infections etc etc.
Vitamin D researchers have recently shown that our bodies use the vitamin d , created by a dose of sunlight, i.e. UVB , or indeed supplements, to read genes in every cell in our body, as and when those cells require. This is an autocrine response. After 2 - 3 days, the vitamin d is converted in the liver, to 25hydroxyD3, the storage form, not so accessible for the autocrine response.
SO,
a monthly dose of UVB is not of maximum benefit, far better to have fresh Vitamin D every few days , or less, such that your body can use it more effectively. Although the storage form ( 25hydroxyD3 ) is used in maintaining the bodies calcium balance ( arguably the most important function - as it keeps your muscles including heart, nerves etc functioning correctly), in order to maximise the benefit you should have the non-converted parent compound D3 circulating in your body at the same time. To achieve this you need more frequent dosing.
IF you decide to go down the sunbed route, then be aware that a mild tan, is your bodies way of protecting the nucleus of each cell, by generating melanin which will congregate above the nucleus , thus protecting it from UV radiation. You should probably have no more than a half MED ( (Minimal Erythemal Dose ) exposure. This means roughly half the exposure which would cause a slight reddening of the skin , the day after exposure. So, a half MED that would show no apparent effects , but will generate Vitamin D under UVB light.
IF you have ginger hair,very pale skin or lots of freckles then you have Type 1 skin, and probably should not be exposing yourself to anything other than a very short, minor dose. NEVER EVER BURN. Daily oral supplements are probably the way to go, or very close to daily.
The darker your skin , the more tolerant you are likely to be. However NEVER EVER BURN, no matter what your skin type.
Your (white ) skin can only make Vit D for 20 mins maximum,when exposed to UVB radiation, any exposure longer than that is not going to make more vitamin D . Your skin has a self regulating mechanism, excess Vit D is broken down in your skin, you cannot become overdosed with vitamin D by sunlight. You cannot wash off vitamin D after sun exposure. ( a common myth ). Darker pigmented skin will require 3 - 5 times the exposure to make the same amount of vit D .
It is a good idea to measure your Vit D level, then using supplements or maybe sunbed etc, raise that level to optimum for health. That level is 100 -150 nmol/l according to vit d researchers worldwide. The level commonly found in this country UK is 30 -70 depending upon time of year , personal exposure habits,skin pigmentation,diet etc etc. If you are below 25, which quite a few people are, your risk of developing many nasty diseases increases dramatically.
I have posted quite a bit about Vit d on various threads here.
Look here for lots of information.
www.mumsnet.com/Talk/general_health/2421492-Vitimin-D-can-it-have-this-effect
also here
www.mumsnet.com/Talk/general_health/2376645-Hospital-wouldnt-test-me-for-vitamin-D
also search under my username for lots more ..!!
Hope this is helpful
BTBH