The most common cause of COPD is long-term smoking or years of exposure to second-hand smoke.

VOA: standard.2010.06.21

Some, like the smallpox vaccine on one exposure generates a very high response that lasts for many years so you don't need a boost.

有一些疫苗,比如接触一次天花疫苗后,人体就能产生高水平的免疫应答,并持续很多年,所以无需再次接种

耶鲁公开课 - 生物医学工程探索课程节选

Barth wrote a famous essay just the year before these stories came out all in one volume. They had been written over a series of years, got a lot of exposure--called "The Literature of Exhaustion."

在写完这一卷故事的前一年,巴斯写了一篇著名的论文，在60年代，叫《枯竭的文学》

耶鲁公开课 - 1945年后的美国小说课程节选

"Over a few years of exposure, the increase for your risk of developing cardiovascular disease...

VOA: standard.2010.08.04