However, no study on the prevalence of infertility has been conducted in this area.
但是，在这个地区不曾进行过任何关于不孕症流行情况的研究。
Objective To study the influence of different coal ranks on the prevalence of pneumoconiosis.
目的探讨不同煤质煤尘对尘肺患病的影响。
DESIGN: a study of epidemiological prevalence.
设计：现况调查。
DESIGN: a study of epidemiological prevalence.
设计：现况调查。
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