Topic 8.13 Notes – Dose Response Curve

What a Dose Response Curve Shows

A dose-response curve connects the dose of a toxin, pollutant, or drug to the response it causes in organisms.

The core idea is simple. Toxicity depends on how much gets into the organism. A substance can be dangerous, but if the dose is tiny, the effect may be small or not detectable.

Dose-response curves can show two different kinds of outcomes:

Graded response in an individual

This is about how severe the effect is in one organism.

Examples include:

• growth inhibition such as slower plant growth
• organ damage such as liver injury
• enzyme disruption that interferes with normal body chemistry
• reproductive impairment such as reduced fertility

Population response

This is about how many organisms in a group show an effect.

Common y-axis measures:

• percent of organisms showing an effect
• percent mortality (the percent that die)

Read the y-axis carefully. A graph can look like a mortality curve but actually show reproductive failure or some other effect. If the y-axis is not death, you cannot treat it like a mortality graph.

The example below uses response (%) on the y-axis, so read it as a general effect curve. It also highlights key thresholds such as NOAEL, LOAEL, and LC50.

Chemical dose-response curve

Dose and Response Variables

On these graphs, dose is the independent variable on the x-axis, and response is the dependent variable on the y-axis.

Dose is often written as mg/kg. That means milligrams of substance per kilogram of body mass. This matters because it lets you compare a small organism and a large organism fairly.

The total amount received is

$$\text{total amount received} = \text{dose per body mass} \times \text{body mass}$$

Dose is not automatically the same as environmental concentration. A pond may have a certain concentration of pollutant, but the actual dose depends on:

• how much water is ingested or inhaled
• how long exposure lasts
• the organism’s body mass

For population data, percent mortality is

$$\text{percent mortality} = \left(\frac{\text{number that die}}{\text{number exposed}}\right)\times 100$$

Response can be measured as percent mortality, percent with a specific effect, severity of effect in one individual, or changes in growth, reproduction, behavior, or body function.

How to Read the Curve

Most population dose-response curves are sigmoidal, which means S-shaped.

Low-dose region

There is little or no observed response.

Middle region

The curve rises steeply. A small increase in dose causes a large increase in response.

High-dose region

The curve plateaus because most or nearly all organisms already show the response.

Threshold and nonthreshold

• Threshold curve means there is an initial range with no observed harmful effect, then the curve rises.
• Nonthreshold curve means risk starts increasing from the lowest doses shown.

Slope

Slope tells you how quickly response changes.

• Steep slope = small dose change causes a big response change
• Gradual slope = response changes across a wider dose range

A common test trap is mixing up slope and position.
- Slope tells how fast response changes.
- Left or right position tells how much dose is needed.

Some graphs use a logarithmic x-axis. On a log scale, equal spacing means multiplying by a constant factor, not adding the same amount each time.

LD50 and Comparing Toxicity

LD50 means the lethal dose that kills 50% of a tested population. It is one point on a mortality curve.

To find it from a graph:

1. Find 50% mortality on the y-axis.
2. Move across to the curve.
3. Move down to the x-axis.
4. Read the dose and units.

Dose-response curves showing ED and LD values

In this graph, LD50 is the point on the right-hand curve where 50% of the population responds lethally.

Lower LD50 means greater acute toxicity. Higher LD50 means lower acute toxicity.

When comparing two substances:

• compare at the same response level
• a left-shifted curve is more acutely toxic
• a right-shifted curve requires more dose
• if curves cross, one may be more toxic at one response level but not another

Comparisons must match:

• same endpoint
• same dose units
• same species or population
• same life stage
• same route of exposure
• same exposure duration and observation period

What Can Change the Curve and What It Can Tell You

Dose-response relationships can change with:

• species
• genetics and individual sensitivity
• age or life stage
• health and body condition
• route of exposure
• exposure duration and frequency
• chemical interactions with other substances

Acute exposure is short-term and is often what LD50 describes.
Chronic exposure is repeated or long-term and can cause effects LD50 does not show.

A graph can support conclusions about:

• whether response increases with dose
• whether a threshold appears present
• where the steep-response region is
• which substance is more acutely toxic under tested conditions

A graph cannot by itself prove:

• real environmental exposure levels
• exact ecological consequences in nature
• universal safety for every species and condition