Methyl parathion is an insecticide. The current US EPA reference dose (RfD) for methyl parathion is 2.5 x 10-4 (0.00025) mg/kg/day. A toxicologist has conducted an experiment (hypothetical) in which six groups of laboratory mice were orally administered different doses of methyl parathion in their diet over a period of 24 months. At the end of the experiment each animal’s hematocrit was assessed. The following table shows the doses of methyl parathion administered to each group and the difference in average hematocrit for that group relative to the average among the control mice. Please review the table, then answer six questions. (11 points total)

Dose Group Dose Administered (mg / kg / day) % Difference in Mean Hematocrit From Control Group

1 (control) 0 —–

2 1 x 10-4 (0.0001) 2%

3 5 x 10-4 (0.0005) – 1%

4 1 x 10-3 (0.001) – 4%

5 5 x 10-3 (0.005) – 19% **

6 1 x 10-2 (0.01) – 32% **

**Indicates a statistically significant difference from the control group.

a. What is the NOAEL for methyl parathion in this experiment?

b. What is the LOAEL for methyl parathion in this experiment?

c. Based on these data, does it appear that there is a “threshold” for the effects of

methyl parathion on hematocrit (yes or no)? Please describe how you arrived at your answer.

d. Can you obtain the LD50 from these data (yes or no)? Please justify your answer.

e. Based on the results of this experiment do you think the RfD should be changed? Why or why not?

f. Your friend who hasn’t taken HSCI 845 wants to know if methyl parathion causes cancer (if they are so interested in environmental health, they should take the class and stop bothering you with these questions!). What would you tell them? Please describe how/why you chose your answer.