Plant growth is adversely affected by chloride levels in the soil falling below 2 parts per million.

Emissions are claimed to be well below 10 parts per million even without exhaust gas recirculation.

Below three parts per million, marine life becomes "severely stressed," he said.

Federal guidelines in the United States consider cyanide levels below 15 parts per million to be safe.

Normal levels are below 71 parts per trillion.

They too found liver damage in test animals from exposures below 500 parts per million (ppm).

The idea is to keep atmospheric CO2 concentrations below 450 parts per million.

The draft plan says that oxygen concentrations should never fall below 1.5 parts per million to avoid lethal effects to marine life.

Nitrate levels must remain below 50 parts per million to stay within the law.

"We found that keeping ozone accumulation below 50 parts per billion is sufficient to protect human health," she said.