To closely evaluate the impact of air pollution on health, the data was split into four 16 year chunks, beginning from 1966 and ending in 1998. Findings showed that black smoke and sulfur dioxide were strongly linked to risks of early death.
Researchers also observed that, although the amount of air pollutants decreased during the study period, the risk of early death (especially from respiratory illness) still remained. For every 10 parts per billion increase in black smoke and sulfur dioxide, the risk of premature death increased by 4% and by 13%, respectively during 1982-1998.
The authors believe the findings confirm the continued strain of air pollution on public health and they point to prevalent health risks even with relatively low levels of black smoke and sulphur dioxide.