What you need to know
- 2.1Ventilation and effective gas exchange.
- 1.15–1.16correlationA relationship in which two variables change together. A correlation does not prove that one causes the other., causationWhen a change in one variable directly causes a change in another. Showing it needs controlled experiments or a known mechanism. and study design.
Original fictional data and a supplied asthma mechanism; no student symptoms or health data are requested.
By the end, you should be able to…
- Explain how airway narrowing can impair ventilation.
- Describe an association without claiming proof of causation.
Build your understanding
A supplied airway model
Asthma can involve airway inflammationA local response to infection or damage: histamine makes blood vessels dilate and become leakier, causing heat, redness and swelling and bringing in phagocytes., narrowing and mucus. Air pollution can trigger symptoms in some people. For this activity, assume narrowed airways reduce ventilation of some exchange surfaces; detailed inflammatory pathways are not required.
Read the invented observations
A fictional study reports symptom episodes on 8 of 100 low-pollution observation days and 20 of 100 high-pollution days. The proportions are 8% and 20%, an absolute difference of 12 percentage points. These are day-based observations, not the percentage of all people who have asthma.
Test an explanation
Reduced ventilation can make maintenance of steep oxygen and carbon-dioxide gradients harder. However, a pollution–symptom association could also be affected by pollen, weather, infection or differences in measurement. Repeated observations of the same person are not automatically independent individuals.
Make your reasoning visible
Keep the denominator visible
Days, people and episodes are different units of observation.
Do not replace evidence with a plausible mechanism
A mechanism supports a hypothesis but does not remove confounding from a study.
Avoid this misconception
The claim: An association between polluted days and symptoms proves pollution is the only cause.
The correction: Several triggers and confounders may contribute; the study design determines what can be inferred.
Check your understanding
Try each question before opening the suggested answer.
Practice question
Retrieval and explanation · suggested answer
Calculate relative frequency of symptom days in the high-pollution group compared with the low-pollution group.
Show suggested answer
20/8 = 2.5 times the observed frequency.
Practice question
Retrieval and explanation · suggested answer
Name one possible confounder and explain why it matters.
Show suggested answer
Pollen may increase on the same days and also trigger symptoms, so it could contribute to the association attributed to pollution.
Take the idea further
Apply the same specification knowledge to an unfamiliar problem. Any extra assumptions are supplied in the question.
Stretch question
A study records 1,000 days from just one participant. Is it equivalent to a study of 1,000 independent participants?
Show a hint
Consider repeated measures and generalisability.
Explore the reasoning
No. Measurements share one person’s characteristics and may be correlated across time. Many observations can improve understanding of that individual without establishing a broadly representative population sample.
Recall this lesson
Lesson recall questions
Question 1 of 5: A headline says ‘study proves X causes heart deaths’ from a single correlation table. What wording is safer?
Illness data: correlation and causation
Practice question
Retrieval and explanation · suggested answer
A headline says ‘study proves X causes heart deaths’ from a single correlation table. What wording is safer?
Lesson: 1C 1: Risk, Correlation and Cause
Try this from memory first.
Put your biology to work
Explore where these ideas appear in the world. Enrichment, not extra required recall.
Joining exposure data to biology
2A · Thinking Bigger: Asthma and Air PollutionNHLBI identifies air pollution among possible asthma triggers. Environmental health research links measurements of exposure with biological outcomes. Suppose a fictional survey finds more reported wheezing near a…Specification 2.1(iii) · 1.15 · 1.16Read & use the scienceClose article
Why this matters
NHLBI identifies air pollution among possible asthma triggers. Environmental health research links measurements of exposure with biological outcomes. Suppose a fictional survey finds more reported wheezing near a busy road. The association suggests a question; it does not establish that traffic pollution caused every symptom. Researchers would need comparable exposure measurements, consistent symptom definitions and information about alternative explanations such as smoking exposure. Narrowed airways can reduce ventilation, connecting the population pattern to a plausible biological mechanism. Keeping the mechanism separate from the strength of the study is essential when interpreting environmental health headlines.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 1, pp. 15–18. The application extends beyond required recall.
- 2.1(iii) understand how the structure of the mammalian lung is adapted for rapid gaseous exchange
- 1.15 be able to analyse and interpret quantitative data on illness and mortality rates to determine health risks, including distinguishing between correlation and causation and recognising conflicting evidence
- 1.16 be able to evaluate the design of studies used to determine health risk factors, including sample selection and sample size used to collect data that is both valid and reliable
Use the science
A fictional survey finds more self-reported wheezing near a busy road than farther away. What measurements and comparisons would help test a pollution explanation, and why could narrowed airways affect gas exchange?
Explore a possible response
Measure pollution over comparable periods and record symptoms consistently in comparable groups. Consider smoking exposure, age and other confounders. Narrowed airways can reduce ventilation and maintenance of the oxygen gradient at exchange surfaces. A plausible mechanism supports investigation but does not remove confounding or prove causation in this survey.
Explore the source
Why wildfire smoke can reach deep into a lung
2A · The Mammalian Gas Exchange SystemWildfire smoke contains particles of different sizes. The smallest particles can reach lower regions of the respiratory tract, including the alveolar region, where the thin exchange surface normally…Specification 2.1(iii) · 2.1(ii)Read & use the scienceClose article
Why this matters
Wildfire smoke contains particles of different sizes. The smallest particles can reach lower regions of the respiratory tract, including the alveolar region, where the thin exchange surface normally supports rapid gas transfer. The EPA describes how deposited particles can provoke inflammation and other biological effects. Exposure depends on particle concentration, duration and breathing pattern, not simply whether smoke is visible. This connects lung architecture with environmental monitoring: a feature that benefits oxygen uptake also places delicate tissue in contact with inhaled material. Evidence about exposure and tissue response is needed before turning an air-quality measurement into a health prediction.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 1, pp. 15–18. The application extends beyond required recall.
- 2.1(iii) understand how the structure of the mammalian lung is adapted for rapid gaseous exchange
- 2.1(ii) understand how the rate of diffusion is dependent on these properties and can be calculated using Fick’s Law of Diffusion
Use the science
Two people spend the same time in air with the same particle concentration, but one is exercising. Why might their inhaled particle doses differ?
Explore a possible response
Exercise can increase ventilation, so a larger volume of air and more particles are inhaled. Deposition also depends on particle properties and breathing route.
Explore the source
- Why is Smoke a Health Concern?US Environmental Protection Agency