2.17 INDOOR INHALATION OF VOLATILE POLLUTANTS
Indoor uses of domestic water will allow volatile
pollutants to escape and cause inhalation exposure. Two models are available
in MEPAS for estimating the risk from indoor inhalation of volatile pollutants:
the MEPAS shower inhalation model and the USEPA Andelman indoor inhalation
model. The MEPAS shower inhalation model is described first, followed by
the USEPA Andelman model.
During showering with domestic water, individuals
may be exposed to airborne volatile pollutants released from the hot shower
water. This exposure pathway is applicable to the groundwater and surface
water transport pathways. As for the drinking water pathway, consideration
is given to reductions of pollutant concentration during processing in
the water supply treatment plant (if present) and in transport through
the water distribution system to the exposed individuals. The surface water
pathway also includes estimation of losses of volatile chemicals in transport
between the point of entry to the surface water and the water-intake plant.
The considerations for this exposure pathway are as follows.
- Transport Medium:
- water concentration at water treatment plant, Cswi or Cgwi, pCi/L or mg/L, expressed as a 70-year average value
- Special Process:
- removal of pollutants during water treatment loss of pollutants (environmental degradation or radioactive decay) during ransport from the treatment plant to the exposure location (households) loss of pollutants during transport in the surface water body by volatilization volatilization of pollutants from the hot shower water to the air inside the shower
- Exposure Factors:
- inhalation rate, shower duration, shower frequency, and exposure duration.
The pollutant concentration reaching the home in
domestic water for shower use is calculated as for the drinking water pathway
described in Subsection 1.2.1. The water concentration is used to estimate
the shower air concentration. Because showering represents a system that
promotes release of volatile chemicals from the water (i.e., high turbulence,
high surface area, and small droplets), the concentration of the contaminant
in the shower air is assumed to be in equilibrium with the concentration
in the water. The concentration in shower air can be estimated using Henry's
law constant (Lyman et al. 1982) as follows:
(91)
where
Csai = concentration of pollutant i in shower air (mg/m3 or pCi/m3)
103 = units conversion factor (L/m3)
Cdwi = concentration of pollutant i at the pumping station or well for domestic water supply (mg/L or pCi/L)
TFi = water treatment purification factor giving the fraction of pollutant, i, remaining after treatment (dimensionless)
lgi = environmental degradation and decay rate constant for closed water system (d-1)
THdw = holdup time in transfer of water from the pumping station or well to the consumer (d)
Hi = Henry's law constant (m3 atm/g-mole)
R = gas law constant (m3 atm/g-mole K?)
T = average absolute water temperature in the shower (degrees Kelvin).
Equation (91) will predict relatively high air concentrations for highly
volatile contaminants; therefore, a mass balance must be performed to ensure
that the amount of contaminant predicted to be in the shower air is not
greater than the total amount in the shower water. The mass balance can
be represented as
(92)
where
Va = volume of air in the shower stall (m3)
Vw = volume of water used during the shower (L)
and other terms are as previously defined. Nominal volumes of 2 m3
and 190 L (about 50 gal) are assumed for the air and water volumes, respectively.
By using these values in Equation (91), and solving for the Henry's law
constant, the maximum allowable Henry's law constant is found to be 2.4
x 10-3 m3-atm/g-mole. The value of the Henry's law
constant is therefore limited to a maximum value of 2.4 x 10-3
in application of Equation (90). The air concentration is used to estimate
the average daily dose for the shower inhalation pathway for groundwater
transport for chemical pollutants, as follows:
(93)
where
Dsii = average daily inhalation dose from chemical pollutant i for the shower inhalation pathway (mg/kg/d)
Usi = inhalation rate while showering (m3/d)
Fsi = fraction of days per year that showering occurs (dimensionless)
FEsh = average frequency of showering events (events/d)
TEs = average duration of each showering event (h/event)
24 = units conversion factor (h/d)
EDsi = exposure duration for the shower inhalation pathway (yr)
BWsi = body weight of individuals exposed via the shower inhalation pathway (kg).
ATsii = averaging time for shower inhalation exposure to pollutant i (yr).
The averaging time for noncarcinogenic chemicals is set to the exposure
duration, and the averaging time for carcinogenic chemicals is fixed at
70 years.
For radionuclide pollutants, the total lifetime dose
is evaluated as follows using the dose conversion factor to convert from
intake to dose (rem).
(94)
where
Dsii = total lifetime ingestion dose from radionuclide i for the shower inhalation pathway (rem)
DFhi = dose conversion factor for inhalation of radionuclide i (rem/pCi ingested) and other terms are as previously defined.
The second model available for estimation of exposure
from indoor inhalation of volatile pollutants is the USEPA model (USEPA
1991) based on work by Andelman (1990). This model uses a factor applied
to the water concentration to estimate the average indoor air concentration
of the volatile pollutant. The considerations for this exposure pathway
are as follows.
- Transport Medium:
- water concentration at water treatment plant, Cswi or Cgwi, pCi/L or mg/L, expressed
as a 70-year average value
- Special Process:
- removal of pollutants during water treatment loss of pollutants (environmental degradation or radioactive decay) during transport from the treatment plant to the exposure location (households) loss of pollutants during transport in the surface water body by volatilization volatilization of pollutants from the hot shower water to the indoor
air, circulated throughout the house
- Exposure Factors:
- inhalation rate.
The pollutant concentration reaching the home in
domestic water for indoor inhalation is calculated as for the drinking
water pathway described in Subsection 2.1. The concentration in indoor
air is estimated using a volatilization factor applied to the water concentration,
as suggested by Andelman (1990). The factor is set to zero for pollutants
for which the following conditions are not met:
Henry's Law Constant >10-5 atm-m3/mole and molecular weight <200 g/mole
When these conditions are met, the factor is set as described in Subsection 5.19.
(95)
The daily intake rate is evaluated from the air concentration
in the home following volatilization of a pollutant from domestic water.
The concentration of chemical pollutants in the air in the home is evaluated
as follows.
where
Ciai = concentration of pollutant i in indoor air from volatilization from domestic water uses (mg/m3)
Cdwi = concentration of pollutant i at the pumping station or well for domestic water supply (mg/L or pCi/L)
TFi = water treatment purification factor giving the fraction of pollutant, i, remaining after treatment (dimensionless)
lgi = environmental degradation and decay rate constant for closed water systems (d-1)
THdw = holdup time in transfer of water from the pumping station or well to the consumer (d)
Kc = Andelman volatilization factor for chemical pollutants (L/m3).
The average daily dose for chemical pollutants is calculated
from the indoor air concentration as follows:
(96)
where
Diai = average daily inhalation dose from chemical pollutant i for the indoor air inhalation pathway (mg/kg/d)
Ciai = concentration of pollutant i in indoor (mg/m3)
Uia = indoor inhalation rate (m3/d)
Fia = fraction of days per year that exposure to indoor air occurs (dimensionless)
EDia = exposure duration for the shower inhalation pathway (yr)
BWia = body weight of individuals exposed via the indoor inhalation pathway (kg)
ATiai = averaging time for shower inhalation exposure to pollutant i (yr).
The averaging time for noncarcinogenic chemicals is set to the exposure
duration, and the averaging time for carcinogenic chemicals is fixed at
70 years.
For radionuclides, the indoor air concentration is
evaluated using Equation (95) with the Andelman factor defined for radionuclides
as follows:
(97)
where
Ciai = concentration of radionuclide i in indoor air from volatilization from domestic water uses (pCi/m3)
Kr = Andelman volatilization factor for radionuclide pollutants (L/m3)
and other terms are as previously defined.
The Andelman factor for radionuclides is applied
to radionuclides that meet the criteria on molecular weight and Henry's
Law constant, and for Rn-222. The total lifetime dose from inhalation of
indoor air is evaluated as follows:
(98)
where
Diai = total lifetime inhalation dose from radionuclide i for the indoor air inhalation pathway (rem)
Ciai = concentration of radionuclide i in indoor air (pCi/m3)
and other terms are as previously defined.