一部分原因在于，癌症的出现往往要经历数十年，因此，要想揭示其导致癌症发生率的增加，必须要对实验对象进行长时间的跟踪。

Part of the problem is that cancer can take decades to appear, so only a lengthy follow-up will expose an increase in its incidence.

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死亡率增加的原因可能是由于雌激素刺激新血管生长——导致癌症发生或扩散。

The reason for the increased mortality could be oestrogen stimulating the growth of new blood vessels - which can cause the cancer to grow and spread.

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背景：动物研究发现联氨可能为肺部致癌物质，但人类研究相当少，且局限于癌症死亡率之调查。

Background: Animal studies suggest that hydrazine is a lung carcinogen, but human studies have been rare, rather small, and limited to cancer mortality.

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