虚线曲线表示背景水平。

Dashed curves are for background levels.

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站到七尺以外的话，你已经非常接近背景水平，也就是说呼吸到正常的空气。

Move seven feet away, and you're "close to background, " i.e., breathing normal air.

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新的安全水平比地平面灰尘背景水平高出一百倍，也比从火山爆发以来测试飞机在英国上空所测的数据要高很多。

The new safety level is about 100 times higher than the background level of dust at ground level.

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