We did not see effect modification by region, even though the background levels of carbon monoxide, NO2, ozone, and particulate matter < 10 μm in aerodynamic diameter varied among the regions, with the San Bernardino area recording higher levels for most pollutants.
In his earliest measurements of the air, taken in California and other parts of the West in the mid-1950s, he found that the background level for carbon dioxide was about 310 parts per million.
The concentration of atmospheric carbon dioxide has increased by 22% since 1960 to a current background level of approximately 385 μmol/mol (Intergovernmental Panel on Climate Change 2007).
The fall-out reaching Scandinavia was only twice the natural background level of radiation.
Spot testing around Chicago showed background levels of one to two micrograms of black carbon per cubic meter.
Major American cities generally have background levels of one to three micrograms of black carbon per cubic meter.
A non-smoker group provided background levels of BoE.
Only background levels of staining were observed (data not shown).
Background levels of genetic alterations.
In 2007 the US-based software giant reached its highest level of carbon emissions.
