Why are dark skinned children who live in smoggy northern cities likely to develop rickets?

Rickets is caused by a lack of vitamin D. Vitamin D is obtained in two ways

1) It is absorbed from foods that you eat

2) It is produced by the skin when it is exposed to sunlight.

The increased pigmentation of the dark skinned children will decrease the amount of sunlight that can be absorbed to produce vitamin D. The smog will also block out some of the sunlight further decreasing the amount of light that the children can use to create vitamin D.

There is also the socioeconomic impact. The children may be unable to obtain adequate nutrition, decreasing the amount of vitamin D they absorb from food.