EPA-Expo-Box (A Toolbox for Exposure Assessors)
- Fate & Transport
- Calculation Tools
Concentrations of contaminants in the air are needed for assessing exposure via the inhalation route (see Inhalation Module in the Routes Tool Set of EPA-Expo-Box). Air concentrations may also be used as data inputs for modeling the fate and transport of contaminants in air, including deposition and/or uptake into other media (e.g., deposition to soil and uptake to vegetation and/or terrestrial animals used as food sources).
Air concentrations expressed in units of mass-per-mass (e.g., ppm or ppb) may be converted to units of mass-per-volume (e.g., mg/m3 or µg/m3) using the ideal gas law and information about the molecular weight of the compound, air temperature, and air pressure. The formulae for performing such conversions are provided here: http://www.epa.gov/athens/learn2model/part-two/onsite/ia_unit_conversion_detail.html.
Characterizing contaminant concentrations for an exposure scenario is typically accomplished using some combination of the following approaches:
- Sampling air and measuring contaminant concentrations
- Modeling the concentrations based on source strength, media transport, and chemical transformation processes
- Using existing, available measured concentration data collected for related analysis or compiled in databases
Information on sampling and analytical methods for measuring concentrations of contaminants in air is provided here. In general, there are two types of air sampling devices used—active and passive.
- Active sampling depends on pumping or similar processes that force movement through an appropriate collection device.
- Passive sampling involves nonmechanical processes like diffusion without any active movement of the medium (U.S. EPA, 2004). In an outdoor environment, passive sampling devices would more likely be influenced by changing environmental conditions such as wind speed and temperature.
Measured concentrations of pollutants in ambient or indoor air could be used with fate and transport models (e.g., dispersion models) or other calculation tools to help characterize exposure.
In the absence of monitoring data, models such as those described in the table below, may be used to estimate the concentrations of contaminants in air. Models may also be used in conjunction with monitoring data to estimate air concentrations at the point-of-contact with receptors, considering fate and transport processes.
Air quality models use ambient measurements or emission factors along with other data and mathematical formulations to estimate concentrations at the point of exposure.
- EPA’s Support Center for Regulatory Atmospheric Modeling (SCRAM) provides access to a variety of models that can be used to “simulate the physical and chemical processes that affect air pollutants as they disperse and react in the atmosphere” along with supporting documentation and guidance.
- EPA's Air Quality Modeling Group (AQMG) provides leadership and guidance to the EPA regions and state/local/tribal agencies on the selection and use of models in regulatory settings.
Indoor air quality models allow users to perform indoor air quality simulations that consider emissions sources, sinks, heating, ventilating, air conditioning (HVAC) systems, and air cleaning devices. Some models are designed to quantify individual exposures and characterize risks associated with indoor air pollutants based on activity patterns and source use.
Under the Clean Air Act, EPA establishes air quality standards to protect public health and the environment. EPA has set national air quality standards for six priority pollutants: carbon monoxide, ozone, lead, nitrogen dioxide, particulate matter, and sulfur dioxide. EPA has created monitoring networks across the country to assess air quality trends for these and other hazardous air pollutants. There are a number of information sources that provide monitoring data on contaminant concentrations in ambient air (see table below). These databases provide ambient air concentrations of contaminants at the national level and for other broad geographic areas such as cities, counties, and states.
Surveys that provide indoor air quality (IAQ) data on a large geographic scale are limited. Two examples are the BASE and TEAM studies.
- The Building Assessment Survey and Evaluation (BASE) Study was conducted to assess indoor air quality from 100 randomly selected public and commercial office buildings located in 37 cities and 25 states. Data were collected on levels of inhalable particles (PM2.5 and PM10), VOCs, formaldehyde, bioaerosols, and radon.
- The Total Exposure Assessment Methodology (TEAM) Study measured exposures to 20–25 VOCs in the air, drinking water, and exhaled breath of over 600 individuals in 4 states. The TEAM researchers concluded that indoor air in the home and at work far outweighs outdoor air as a route of exposure to toxic chemicals, and these elevated indoor levels appear to be associated with building materials, consumer product use, and personal activities such as smoking.
In addition, information from EPA is available that provides general pollutant concentration levels believed to be “typical” in the home (in some instances compared with expected outdoor air levels) for VOCs, lead, nitrogen dioxide, formaldehyde, carbon monoxide, and respirable particulates (see links to specific air pollution sources at http://www.epa.gov/iaq/ia-intro.html).