Why might a scientist studying water pollution work with percents less then one?

The majority of pollutants reach levels that cause environmental impacts at part per million levels, this is 0.00001%. These levels, if expressed as percents, would require extensive careful reading to get correct and present oportunites for typogtaphical errors. AT lower levels such as part per billions this problem becomes even more evident.

Thypical pollutant level limits for metals are generally 1 ppm, for phenols 15 ppb.