8.13 - Dose Response Curve

Learning Objectives

I. Plotting the Dose Response Curve

• Dose response curves plot the data from a dose response study.

• Percent mortality, or whatever effect is being studied, goes on the Y-axis.

• Dose concentration, the independent variable the scientist manipulates, goes on the X-axis.

• Curves typically follow an S-shape. Mortality is usually low at low concentrations, ramps up rapidly after the threshold, then levels off as it approaches 100%.

• The shape happens because different organisms have different tolerance levels.

II. Key Terms on the Curve

• Threshold: The lowest detectable level where a response shows up in the organism. For example, the threshold might be 40 milligrams per kilogram, where some organisms in the study started to die.

• $LD_{50}$: The lethal dosage that kills 50% of the organisms in the study.

• Reading $LD_{50}$ from the graph: Find 50% on the y-axis, go across to the curve, then straight down to the x-axis. On the graph above, the $LD_{50}$ is 100 mg/kg. The curve is steepest around the $LD_{50}$, so a small increase in dose there causes a big jump in deaths.

III. $ED_{50}$ (Non-Lethal Effects)

• $ED_{50}$ is the concentration at which 50% of the organisms in a study show some sort of response that’s a non-lethal effect.

• Sublethal effects are outcomes that don’t kill the organism but are measurable and quantifiable. Examples include infertility, paralysis, or developing cancer or a tumorous growth.
• A great example of $ED_{50}$ is the concentration of the herbicide atrazine that causes 50% infertility in frog populations.

• When you compare lethal ($LD_{50}$) and non-lethal ($ED_{50}$) responses, the $LD_{50}$ curve is generally shifted further to the right, meaning a higher dose, than the $ED_{50}$ curve.

IV. Study Limitations

• Most dose response studies are acute (short-term), so they may not capture long-term developmental issues.

• Chronic studies are longer term, sometimes following an organism from hatching all the way through sexual maturity and adulthood.

V. Comparing Two Chemicals

• Farther left = more toxic: The curve that sits farther to the left reaches 50% mortality at a lower dose. Here, chemical A’s $LD_{50}$ is 30 mg/kg and chemical B’s is 100 mg/kg, so A is about 3.3 times as toxic as B ($100 \div 30 \approx 3.3$).

• Same dose, different results: At a dose of 50 mg/kg, chemical A kills 100% of the test organisms, while chemical B kills almost none (under 1%).

• Thresholds: Chemical A starts killing organisms at about 10 mg/kg, but B doesn’t until about 40 mg/kg, so a regulator would set a much lower safe limit for A.