For nicotine, that level was a mere 1.8 nanograms per milliliter — too low to be picked up in tests.

对于尼古丁，仅仅是1.8毫微克每毫升，太低而不易提取。

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Optimal levels range from 30 nanograms per milliliter to 40 nanograms per milliliter, the researchers said.

研究人员说，最佳水平范围是每毫升30到40纳克。

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According to the American Cancer Society, most men have levels under 4 nanograms per milliliter of blood (ng/ml).

根据美国癌症协会的说法，大多数男性PSA含量为4ng/ml。

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In the U.S. vitamin D levels are usually reported with a different measure using nanograms per milliliter.

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And from this year, new facilities are to meet the tougher international standard of 0.1 nanograms per cubic meter.

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Reporting in the journal Marine Drugs, the authors found BMAA concentrations ranging from 144 to 1, 838 nanograms per milligram.

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