Case Study Exercise

Print
Related Pages

In PHAT modules, you learned about various steps of the public health evaluation process. Now, use what you’ve learned from the Practice to calculate an EPC for the independent case study exercise. This exercise focuses on working with the arsenic data for soil at the daycare center.

As a reminder, the steps before this Step 3 included:
  • Step 1: Define exposure units to evaluate. You defined a single exposure unit, being the entire outside area of the daycare center.
  • Step 2: Compile data for each exposure unit. For this example, we are focusing on arsenic, the one potential COC that exceeded its CV during the screening and one exposure pathway, soil ingestion. The discrete surface soil sampling data for arsenic are summarized in this table.
Surface Soil Sampling Results for Off-Site Daycare Center
Sample Location Date Sampled Arsenic (mg/kg)
SS-GL-DC-1 12/12/15 1.3
SS-GL-DC-2 12/12/15 350
SS-GL-DC-3 12/12/15 400
SS-GL-DC-4 12/12/15 2.6
SS-GL-DC-5 12/12/15 31
SS-GL-DC-6 12/12/15 350
SS-GL-DC-7 12/13/15 560
SS-GL-DC-8 12/13/15 280
SS-GL-DC-9 12/13/15 290
SS-GL-DC-10 12/13/15 550
SS-GL-DC-11 12/13/15 520
SS-GL-DC-12 12/13/15 460
SS-GL-DC-13 12/14/15 840
SS-GL-DC-14 12/14/15 1,020
SS-GL-DC-15 12/14/15 950
SS-GL-DC-16 12/14/15 430
SS-GL-DC-17 12/14/15 360
SS-GL-DC-18 12/14/15 270
Now you will use the data you compiled in Step 2 to estimate an EPC for arsenic in soil at the single exposure unit at the daycare center.

ATSDR has different guidance based on the type of samples in a dataset that you need to follow to estimate EPCs. For this example, we will follow the EPC Guidance for Discrete Sampling. Always make sure to refer to the guidance that applies to your site-specific data.

Choose the correct answer that relates to calculating the EPC for this exercise:
  1. Calculate an EPC for acute exposures only.
  2. Calculate the 95UCL as the EPC for pica (acute/intermediate) initially.
  3. Use the max concentration for the EPC to assess chronic exposures because there are too few samples.
  4. Use the 95UCL as the EPC to assess chronic exposures because arsenic was analyzed in ≥ eight samples, detected in ≥ four samples, and detected in ≥ 20% of samples.
The correct answer is “D.”

Option “A” is incorrect because based on the site information in Step 1, you learned that there are pica (acute and intermediate) and chronic exposures to evaluate.

Option “B” is incorrect because you learned that the maximum concentration is used as the EPC for pica.

Finally, option “C” is incorrect because as you saw in Step 2, there are 18 samples in the data set.

We will help you a bit by listing a few other items you’ll need to know.
  • You will be evaluating a soil-pica scenario. As ATSDR guidance states, the first step is to run a default pica scenario in PHAST using the maximum concentration as the EPC. PHAST is ATSDR’s public health assessment site tool that health assessors can use when performing their PHA projects. Contact phast@cdc.gov to gain access to PHAST. Based on the site data for this case exercise, the maximum arsenic concentration in soil is 1,020 mg/kg.
    • Note: In this first step, the default scenario in PHAST assumes an exposure of three days a week and an intake rate of 5,000 mg/kg for acute and intermediate scenarios. We won’t do this for our exercise, but at a real site, if the default pica scenario using the maximum concentration is a health concern, you would perform a second step to determine if a single pica event is a concern. This involves entering site-specific parameters, if available, in PHAST. However, site-specific data are not often available and are not required to evaluate this one-time dose. Typically, this one-time dose uses a one-day exposure and a default intake of 5,000 mg/kg.
  • For a real site, you would use the ATSDR EPC Tool to calculate the 95UCL for our chronic exposure. For this exercise, we will do this for you.

Based on our review of the discrete sampling guidance, we know we are calculating the 95UCL to assess chronic exposures. We are using the maximum concentration to assess soil-pica exposures (acute and intermediate) for this exercise.

We will use ATSDR’s EPC Tool for the 95UCL calculation, like we did during our practice exercise.

The output from ATSDR’s EPC tool will show the EPC value (774.6 ppm) and EPC type (95% UCL of the mean) along with flags associated with the sampling data. You will use the EPC value in ATSDR’s public health assessment site tool (PHAST) to calculate hazard quotients (HQs) and cancer risk estimates if health guidelines and cancer toxicity values are available.

Need alt text

You finished practicing calculating EPCs. Let’s go to the next and last primer to learn about the toxicological evaluation.

Back Next