The Canadian

Centre for Environmental Modelling and Chemistry

FHX Model

Version 1.03 - February 2011

New in version 1.03

Software Description

The Farfield Human Exposure (FHX) Model is a screening level assessment model that brings together information on chemical partitioning, reactivity, environmental fate and transport, bioaccumulation, and exposure.

The environmental fate of a set of chemicals is calculated using Level III (steady-state) equations for a unit emission rate. Environmental persistence and potential for long-range transport are calculated for each chemical. A bioaccumulation food web model calculates the exposure experienced by a variety of wildlife, agricultural crops and livestock. Human exposure is then calculated for a specific diet and set of age groups following Health Canada Exposure factors.

This model is intended to be used to compare hazard between a large set of chemicals.

Although the model will address ionizing substances, its treatment of these substances is very simplistic and the user is cautioned that the estimation of partitioning, reaction and bioaccumulation of the ionic species can be in considerable error. The model is primarily designed to address non-ionizing substances.

It is hoped that this model will be useful for identifying and prioritizing those chemicals of commerce that are of greatest potential concern. Following such an assessment, selected chemicals can be more fully and accurately evaluated using monitoring data in conjunction with models describing chemical fate and effects in more detail in real regional environments, food webs, and human populations with specific attention to populations with diets that result in high exposure.

For more information Level III calculations, refer to the description of the Level III model. FHX is similar in structure to the RAIDAR model.


  • Allows the comparison of many chemicals from a file. The file should be prepared by the user in advance and in accordance with the file preparation guidelines.
  • Provides a database of environments and environmental properties that may be added to or modified by the user.
  • Provides context-sensitive Help.
  • Displays and prints the model calculations, as performed by the program.
  • Allows the program results to be saved as a comma separated (csv) text file viewable by most standard spreadsheet software.
  • Printing of the model results is NOT permitted in this program due to the large amount of data produced. It is recommended that the results be saved and analysed. The file may be more usefully printed after analysis is complete.

    This program is based on the following publication:

    Arnot JA, Mackay D, Sutcliffe R, Lo B. 2010. Estimating farfield organic chemical exposures, intake rates and intake fractions to human age classes. Environmental Modelling and Software 25(10):116-1175

    The required input data are:

    Chemical Properties:
    A comma separated value (.csv) file containing the following data in the order given below and with the restrictions as listed. A maximum size of a chemical set is 252 chemicals. Chemicals may be entered directly into the FHX software but this is not recommended unless the set is very small.

    1. There must be exactly 29 columns (A to AC) in the file.

  • CAS (not required)
  • Chemical Name
  • Chemical Type
  • Molar Mass
  • Data Temperature (C)
    Changing the data temperature affects only the Z value in air. None of the other chemical properties are temperature corrected by the model. If the environment temperature is different from the temperature at which the chemical data were collected, the chemical properties must be manually corrected for this temperature dependence.
  • Reaction half-lives in air, water, soil, sediment
  • Metabolic degradation half-lives for fish, and for mammals and birds
  • for Type 1 chemicals:
    - Water Solubility (g/m3)
    - Vapour Pressure (Pa)
    - LogKow
  • Type 2 chemicals
    - Kaw (air-water)
    - K Soil-Water (L/kg)
    - K Sediment-Water (L/kg)
    - K Suspended Particles-Water (L/kg)
    - K Fish-Water (L/kg)
    - K Aerosols-Water
  • Type 3 chemicals
    - K Water-Air
    - K Soil-Air
    - K Sediment-Air
    - K Suspended Particles in Water-Air
    - K Fish-Air
    - K Aerosols-Air
  • Estimated Total Emission kt/y
  • pKa only for ionizing Type 1 chemicals
  • Chemical Name
    where K is a partition coefficient with units as indicated or as a unitless ratio. For each chemical Type, only the partitioning data specific to that Type is required.

    2. Each chemical must have a new row.

    3. The Chemical Name must appear in both the second and last column.

    4. The first row must contain the headings for the columns and the chemical data must start on the second row of the file.

    5. Blank rows are not permitted.

    6. Major modifications to the chemical data should not be done in the FHX model, but should be made in the spreadsheet program of your choice, the file saved in csv format, and imported. The FHX model is designed to handle only minor modifications to the chemical information and will not perform well for major changes or additions.

    Environmental Properties:

  • total human population
  • areas and depths for all bulk media
  • volume fractions for all subcompartments
  • densities for all subcompartments
  • organic carbon content (for soil, sediment, and suspended particles)
  • fish lipid content
  • advective flow residence times for air (including aerosols), and water (including suspended particles and aquatic biota)
  • advective flow residence time for sediment burial
  • air temperature and wind speed (wind speed is used to calculate the long-range transport potential in air, not residence time in the air compartment)
  • water temperature, pH, and water oxygen saturation
  • transport velocities
    - air side air-water mass transfer coefficient
    - water side air-water mass transfer coefficient
    - rain rate
    - aerosol dry deposition velocity
    - soil air phase diffusion mass transfer coefficient
    - soil water phase diffusion mass transfer coefficient
    - soil air boundary layer mass transfer coefficient
    - sediment-water mass transfer coefficient
    - sediment deposition velocity
    - sediment resuspension velocity
    - soil water runoff rate
    - soil solids runoff rate
  • scavenging ratio

    Biota Properties:
    A single set of biota properties are provided in the model. They can not be changed by the user.

    Human Properties:
    A single set of human properties are provided in the model. Only the Food Sources matrix can be changed by the user but any changes will not be saved.

    The mode-of-entry must be specified, all chemicals in the set will be evaluated using this mode-of-entry.

    Model output includes:

  • all input parameters including chemical, environmental, and biota properties, emissions and mode-of-entry, and all parameters derived from these inputs
  • a Level III description of the fate of all the chemicals in the set (including concentrations in biota)
  • human exposure
  • a diagram of enviromental fate

    Notes on Environmental Properties:
    The fish volume fraction and the fish lipid content are required here to maintain continuity with previously exisiting CEMC models such as EQC. They modify, very slightly, the total volume and the Z of the water column. The fish lipid content is used to calculate the fish-water partition coefficient (KFW) in the case of a Type 1 chemical, and the octanol-water partition coefficient (from the KFW) in the case of a Type 2 or 3 chemical. They are not used directly in the foodweb calculations.

    Minimum system requirements:

    Minimum system requirements are an IBM-compatible PC running Windows 98 or XP. This model will not run under Windows NT or 2000.
    The screen resolution should a minimum of 800x600 pixels.

    FHX Version 1.02, released May 2010 continues to be available.
    FHX Version 1.01, released December 2008 continues to be available.
    FHX Version 1.00, released November 2008 continues to be available.

    Please read the FHX SOFTWARE LICENSE before downloading the software.
    Use of the software constitutes your agreement to abide by the terms and conditions set out in the license agreement.

    Bug Report


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    Last updated June 10, 2013.