In general, the institute said, average atmospheric concentrations are 1 to 10 micrograms per cubic meter.

Normal average concentration in clean air is about 0.0001-0.0002 ppm.

Some 860,000 people receive water with an average nitrate concentration above 50 mg/litre.

- The average chlorophyll-a concentration is 4 ug/l, based on 5 years of data.

The average concentration of cadmium in the Earth's crust is between 0.1 and 0.5 parts per million (ppm).

The average concentration is 900 milligrams per liter.

They found that average concentrations across Europe would rise to more than double the background average over the first three months of the eruption.

Chromium is the 24th most abundant element in Earth's crust with an average concentration of 100 ppm.

By comparison the average concentration worldwide is 1,100ppm.

The Lawther report said that the annual average concentration in air should not exceed 2 g/cu.m.