Measurements of haze, smoke, dust, and visibility in the midwest available from 1901 to 1980 reveal sizable temporal fluctuations. These appear useful as surrogates of air pollutants not measured before 1950. Their historical record reflects both fluctuations in climate conditions and changing sources of major pollutants (such as the shift from coal to oil or gas as the primary fuels during 1950s, and the regionally extensive vehicular traffic that had become a major pollutant source since World War II). Regional uniformity found since the 1950s reflects integration of both area and point sources, and a general decrease of visibility from 1950 to 1970. Air quality regulations may have had little affect on haze, smoke, and dust except at very large cities where improvements in visibility have occurred since 1970.
1 Also affiliated with the Illinois State Water Survey.