7
Quantifying Mobility
INTRODUCTION
The impact of a chemical release is dependent not only on the toxicity of the compound but also on the likelihood that we will ingest or inhale the toxin. The impact
of a particular chemical increases proportionally to the mobility of the chemical, that
is, its tendency to dissolve and enter the water we drink or evaporate and enter the
air that we breathe.
The tendency of a chemical to dissolve in a liquid solvent (e.g., water), to form
a homogeneous solution is its solubility. Solubility is dependent on the chemical
itself as well as temperature and pressures. The extent of the solubility of a specific
chemical in a specific solvent is measured as the saturation concentration, for which
adding more chemical does not increase the concentration of the solution. The tendency of a chemical to evaporate is measured by its vapor pressure (VP) at typical
temperature or approximately 70°F. VP is the pressure of a vapor in equilibrium
with its nonvapor phases or the pressure at which the gas of that substance is in
dynamic equilibrium with its liquid or solid forms, an indication of the evaporation
rate of a liquid. A substance with a high VP at normal temperatures is referred to
as volatile. Water, as all liquids, starts to boil when its VP reaches its surrounding
pressure. At higher elevations, the atmospheric pressure is lower, and water will
boil at a lower temperature.
This chapter presents the development of a chemical-specific mobility factor
(MF) (or a factor that represents the tendency of a chemical to dissolve in water and
evaporate into air) based on the solubility of the chemical (in milligrams per liter)
and VP (in atmospheres).
AIR MOBILITY FACTOR
The VP (in atmospheres) of the Toxics Release Inventory (TRI) chemicals was
obtained from the Pennsylvania Department of Environmental Protection. For the
purposes of this analysis, the VPs were set such that none of the VPs was set at more
than 1 atm (anything greater than 1 atm does not have an additional impact with
respect to release to the air), and the minimum was 1 × 10 −9 atm (such that some
dissipation affect is always taken into account). The chemical-specific air MFs, that
is, the tendency for the chemical to evaporate and enter the air that we breathe, were
derived by taking the square root of each chemical-specific VP (in atmospheres)
to narrow the range of the values. Without this adjustment, the VPs would adjust
the mobility of chemicals by a range of over 1 million to 1, which was too much of
91
Quantifying Mobility
INTRODUCTION
The impact of a chemical release is dependent not only on the toxicity of the compound but also on the likelihood that we will ingest or inhale the toxin. The impact
of a particular chemical increases proportionally to the mobility of the chemical, that
is, its tendency to dissolve and enter the water we drink or evaporate and enter the
air that we breathe.
The tendency of a chemical to dissolve in a liquid solvent (e.g., water), to form
a homogeneous solution is its solubility. Solubility is dependent on the chemical
itself as well as temperature and pressures. The extent of the solubility of a specific
chemical in a specific solvent is measured as the saturation concentration, for which
adding more chemical does not increase the concentration of the solution. The tendency of a chemical to evaporate is measured by its vapor pressure (VP) at typical
temperature or approximately 70°F. VP is the pressure of a vapor in equilibrium
with its nonvapor phases or the pressure at which the gas of that substance is in
dynamic equilibrium with its liquid or solid forms, an indication of the evaporation
rate of a liquid. A substance with a high VP at normal temperatures is referred to
as volatile. Water, as all liquids, starts to boil when its VP reaches its surrounding
pressure. At higher elevations, the atmospheric pressure is lower, and water will
boil at a lower temperature.
This chapter presents the development of a chemical-specific mobility factor
(MF) (or a factor that represents the tendency of a chemical to dissolve in water and
evaporate into air) based on the solubility of the chemical (in milligrams per liter)
and VP (in atmospheres).
AIR MOBILITY FACTOR
The VP (in atmospheres) of the Toxics Release Inventory (TRI) chemicals was
obtained from the Pennsylvania Department of Environmental Protection. For the
purposes of this analysis, the VPs were set such that none of the VPs was set at more
than 1 atm (anything greater than 1 atm does not have an additional impact with
respect to release to the air), and the minimum was 1 × 10 −9 atm (such that some
dissipation affect is always taken into account). The chemical-specific air MFs, that
is, the tendency for the chemical to evaporate and enter the air that we breathe, were
derived by taking the square root of each chemical-specific VP (in atmospheres)
to narrow the range of the values. Without this adjustment, the VPs would adjust
the mobility of chemicals by a range of over 1 million to 1, which was too much of
91