In an overview of the updated free modeling tool IH Mod 2.0 at AIHce EXP 2019, industrial hygienists learned about the expanded capabilities and some of the limitations of the popular freeware. Originally launched in 2008, IH Mod is a downloadable mathematical modeling Excel spreadsheet used for estimating occupational airborne exposures. It is available in multiple languages and includes a separate support file. IH Mod 2.0 can be found on the AIHA website.

“IH Mod 2.0 has a slightly different interface, but the same set of equations,” explained presenter Pamela R.D. Williams, MS, ScD, CIH. There are eleven models contained within IH Mod, some of which, Williams said, have slightly different names than they did in the original version. The eleven models fall under the following categories:

Well-Mixed Box Room Model: This model estimates air contaminant concentrations in a room that has well-mixed air. It is useful when contaminant sources are distributed in a workspace with good air mixing, in small spaces with mixing, or when workers are located some distance from a source. IH Mod includes four well-mixed box room models that include equations for constant emission rate, decreasing emission rate, with back pressure, and purging.

Two-Zone Model: The two-zone model estimates air contaminant concentrations near the source (near field) and elsewhere in the work space (far field). It is useful when workers are in close proximity to the contaminant source, but it assumes perfect mixing within each zone and limited airflow between the zones. IH Mod includes two two-zone models that include the constant emission rate and decreasing emission rate equations.

Near- and Mid-Field Plume Model: The near- and mid-field plume model estimates order-of-magnitude air contaminant concentrations downwind for a low-pressure release on the plume centerline in outdoor conditions. It is useful when workers are at distances of up to three meters from a contaminant source such as leaking process equipment (near field) or at the fence-line up to 100 meters (mid-field).

Eddy/Turbulent Eddy Diffusion Model: The eddy/turbulent eddy diffusion model estimates air contaminant concentration gradients with distance as a contaminant diffuses away from the point source. It is useful when workers are at varying distances from the contaminant source or if there are turbulent airflow patterns in the vicinity of the emission source. IH Mod includes four eddy/turbulent eddy diffusion models with the following equations: without advection and constant mass emission rate, with advection and constant emission rate, without advection following pulse release, and with advection following pulse release.

The most significant change to IH Mod 2.0 is the addition of a probabilistic method and the ability to run Monte Carlo Simulations (MCS). IH Mod 1.0 featured only a deterministic method. “A deterministic approach would literally just be using a single value for each of the inputs into the exposure model,” Williams explained. “What IH Mod 2.0 allows you to do is to choose, in addition to a deterministic approach, a probabilistic approach. Now, instead of a single value for each of those inputs, you can choose an entire distribution, and you can specify which distribution.” For example, for certain parameter inputs within each model, the user can select a normal distribution, lognormal distribution, triangular distribution, or uniform distribution to represent that input.

“With the Monte Carlo Simulation, we're trying to account for the variability and the uncertainty in some of these parameters and inputs,” Williams said. The MCS is a probabilistic method that uses random numbers and computer simulations to combine multiple probability distributions. During the simulation, values of variables are selected randomly from each distribution to estimate exposure. This process is repeated by the number of iterations the user chooses to generate a series of exposure estimates. As Williams noted, “When you do a Monte Carlo assessment, you get a whole distribution. Then you get to decide what value on that distribution you want to use to represent exposure. Of course, it depends on the question you're asking. Do you want to know what the average exposure is in the population? Do you want to know who the most highly exposed people are, or what their exposure value is? The choice is something that you have to make given the question you're trying to answer.”

Another subtle change in IH Mod 2.0 relates to some of the input parameters themselves. IH Mod 2.0 includes more behind-the-scenes equations that do some of the conversion work for you. For example, in the two-zone model, the user now specifies the near-field geometry (for example, sphere, box, cylinder) and near-field radius, and the model uses this information to calculate the near-field volume. In the original version, the user had to include a value for the near-field volume. Another important example pertaining to the two-zone model is that the user now specifies the random air velocity, which the model uses along with the calculated free surface area to calculate the interzonal airflow rate. IH Mod 1.0 required the user to include a value for the airflow rate between the near and far field, which was a frequent source of confusion. “This is meant to be more user friendly and more intuitive, the newer version,” Williams said. “If you're used to IH Mod 1.0 and you jump over to 2.0, just be aware you might have to think differently about some of the inputs.”

But the presenters were careful to point out that models are not perfect. “Essentially all models are wrong, but some are useful,” said presenter Alan Rossner, PhD, CIH, FAIHA, repeating a famous quote from the British statistician George Box. “You've got to put in the right variables and, even then, they may be estimates,” Rossner said. “They're simplifications of the real world.”

Other limitations of IH Mod 2.0 include the restriction that distributions are not available for all model parameters; it requires some prior calculations and conversions (such as for emission rate); sources of data for specific input values are often limited; and modeled predictions are an order of magnitude.

Still, the popular IH tool is widely used and known for its many strengths, including the fact that the updated version now allows for a range of input values, provides percentile exposure estimates, accounts for variability and uncertainty, and identifies gaps and research needs.

For those interested in learning more about IH Mod 2.0, a professional development course on the tool will be presented at AIHce EXP 2020 on Sunday, May 31.