When it comes to environmental science, assessing pollution seems to be one of the main research areas at present, simply because of the growing impact it has on the living organisms including the human beings. Although humans are able to tolerate environmental pollution to a certain extent without threat to life, certain organisms are so sensitive to even the slightest change in the ecosystem which will make them perfect ‘bio indicators’ of environmental pollution. Their ability to tolerate pollution has been measured by the biologists and the term ‘tolerance value’ has been used to describe this numerical figure of tolerance.
In environmental science, ‘tolerance’ means the degree to which organisms can withstand environmental degradation. In this regard, not all organisms are able to tolerate a certain level of environmental degradation or pollution at the same intensity and therefore the tolerance levels can be described as a scale in which some organisms can survive even in highly degraded environments while the organisms at the other end could vanish even with the slightest change.
According to Putman and Wratten (1984), ‘Tolerance refers to the breadth of the occurrence of an organism along an environmental gradient.’ However, it has also been interpreted differently for the sake of studying environmental pollution and one such description states the tolerance as representing ‘its [an organisms] optimal position on a gradient of disturbance rather than the breadth of its occurrence.’
The assignment of a ‘tolerance value’ to a particular group of organisms had been there for many years and has a widespread use. These values are often used to assess the water quality in a particular site. Based on these values for a particular organism or group of organisms, the scientists are able to measure the level of pollution, especially with regard to industrial water contamination, and issue quality certificates and recommendations to the local authorities.
Another important aspect of pollution tolerance value is, understandably, it can be different from one organism to another or from one class of organisms to another class. According to some experts, the tolerance values used for the same organism in different states in US is different from one state to another. However, because the final assessment of the environment is dependent upon a matrix of such values, out of which one of the commonest is the Hilsenhoff Biotic Index (HBI), the total outcome seems not to deviate much between different states.
But scientists are of the opinion that the rationale behind some of these values may not be conducive and acceptable to better quality assessment of certain environments and therefore should be reviewed with the best scientific evidence.