It is also considered reasonable to treat at-risk persons with vitamin D supplementation without checking the level of 25(OH)D in the serum, as vitamin D toxicity has only been rarely reported to occur.Levels of 25(OH)D that are consistently above 200 nanograms per milliliter (ng/mL) (or 500 nanomoles per liter, nmol/L) are potentially toxic. Vitamin D toxicity usually results from taking supplements in excess. Hypercalcemia is often the cause of symptoms,  and levels of 25(OH)D above 150 ng/mL (375 nmol/L) are usually found, although in some cases 25(OH)D levels may appear to be normal. Periodic measurement of serum calcium in individuals receiving large doses of vitamin D is recommended. Screening
The official recommendation from the United States Preventive Services Task Force is that for persons that do not fall within an at-risk population and are asymptomatic, there is not enough evidence to prove that there is any benefit in screening for vitamin D deficiency. Treatment
UVB exposure
Vitamin-D overdose is impossible from UV exposure: the skin reaches an equilibrium where the vitamin degrades as fast as it is created. Sun tanning
Light therapy
Exposure to photons (light) at specific wavelengths of narrowband UVB enables the body to produce vitamin D to treat vitamin D deficiency. Supplement
In the United States and Canada as of 2016, the amount of vitamin D recommended is 400 IU per day for children, 600 IU per day for adults, and 800 IU per day for people over age 70.
Instead, wearing clothing is more effective at reducing the amount of skin exposed to UVB and reducing natural vitamin D synthesis. Clothing that covers a large portion of the skin, when worn on a consistent and regular basis, such as the burqa, is correlated with lower vitamin D levels and an increased prevalence of vitamin D deficiency.Regions far from the equator have a high seasonal variation of the amount and intensity of sunlight. In the UK, the prevalence of low vitamin D status in children and adolescents is found to be higher in winter than in summer. Lifestyle factors such as indoor versus outdoor work and time spent in outdoor recreation play an important role. Additionally, vitamin D deficiency has been associated with urbanisation in terms of both air pollution, which blocks UV light, and an increase in the number of people working indoors. The elderly are generally exposed to less UV light due to hospitalisation, immobility, institutionalisation, and being housebound, leading to decreased levels of vitamin D.

Darker skin color
Because of melanin which enables natural sun protection, dark-skinned people are susceptible to vitamin D deficiency. Three to five times greater sun exposure is necessary for naturally darker skinned people to produce the same amount of vitamin D as those with white skin. Malabsorption
Rates of vitamin D deficiency are higher among people with untreated celiac disease, inflammatory bowel disease, exocrine pancreatic insufficiency from cystic fibrosis, and short bowel syndrome, which can all produce problems of malabsorption.