IB DP Environmental Systems and Societies – Human populations, urban systems and urban air pollution Revision Notes

For full explanations and worked examples, start with the Human populations and urban systems study guide. These notes are for fast recall in the final weeks.

They cover Topic 8, Human populations and urban systems, of IB Diploma Programme Environmental Systems and Societies, aligned to the International Baccalaureate Diploma Programme Subject Brief: Environmental systems and societies, first assessment 2026. They cover syllabus sections 8.1 (human populations), 8.2 (urban systems and urban planning) and 8.3 (urban air pollution), studied at both SL and HL (9 hours SL, 15 hours HL, so HL goes into more depth). It follows the IB ESS subject brief for first assessment 2026 — the course examined in the May and November 2026, 2027 and 2028 sessions.

The brief gives Topic 8 Human populations and urban systems 9 teaching hours at SL and 15 at HL. It does not list the subtopics or learning outcomes for this topic, so the numbered subtopics and outcomes on this page follow the syllabus numbering used in the printable ESS checklist, not the brief itself.

Test yourself afterwards with the practice questions. The IB DP ESS course hub lists every topic, and the printable syllabus checklist lets you tick them off. For how the papers work, see the ESS assessment revision notes.

Key definitions

• Crude birth rate (CBR): live births per 1,000 people per year.
• Crude death rate (CDR): deaths per 1,000 people per year.
• Natural increase rate (NIR): growth from births minus deaths, as a percentage. Excludes migration.
• Total fertility rate (TFR): average number of children per woman over her lifetime at current rates. Replacement level ≈ 2.1.
• Doubling time: years for a population to double at a constant growth rate.
• Dependency ratio: dependants (0–14 and 65+) per 100 people of working age (15–64).
• Population momentum: continued growth after fertility falls to replacement, because many young people are still entering child-bearing age.
• Urbanisation: increase in the proportion of people living in urban areas.
• Urban sprawl: low-density spread of a city into surrounding rural land.
• Urban heat island: higher temperatures in a city than in the surrounding countryside, most marked at night.
• Primary pollutant: emitted directly from a source (CO, NOx, SO₂, particulates, VOCs).
• Secondary pollutant: formed by reactions between primary pollutants in the atmosphere (tropospheric ozone, PANs).
• Photochemical smog: a mix of ozone, PANs and other pollutants formed when sunlight acts on nitrogen oxides and VOCs.
• Thermal inversion: a layer of warm air above cooler surface air that stops pollutants rising and dispersing.

Formulas

Quantity	Formula	Units
CBR	births ÷ population × 1,000	per 1,000 per year
CDR	deaths ÷ population × 1,000	per 1,000 per year
NIR	(CBR − CDR) ÷ 10	% per year
Doubling time (rule of 70)	70 ÷ growth rate (%)	years
Dependency ratio	(0–14 + 65+) ÷ (15–64) × 100	dependants per 100 workers
Future population	P × (1 + r)ⁿ, with r as a decimal	people
Percentage change	(new − old) ÷ old × 100	%
IPAT model	Impact = Population × Affluence × Technology	—

Method in steps

Demographic data question

1. Convert raw births and deaths to per 1,000 before anything else.
2. NIR = difference ÷ 10. Write the % sign.
3. Doubling time = 70 ÷ NIR. Round sensibly (whole years).
4. State the assumptions: constant rate, no migration.

Describing a population pyramid

1. Name the shape: expanding, stationary or contracting.
2. Quote at least two figures from the axis (for example, “about 16% of males are aged 0–4”).
3. Point out any anomaly (notch or bulge) and suggest a cause.
4. Link to a DTM stage and to one environmental or social implication.

Explaining photochemical smog

1. Source: traffic emits NO and VOCs, mostly at rush hour.
2. NO oxidises to NO₂.
3. Sunlight: NO₂ → NO + O; then O + O₂ → O₃.
4. VOCs + nitrogen oxides → PANs and more ozone.
5. Conditions: sunny, warm, still weather; an inversion traps the smog.

Evaluating a management strategy

1. Place it on the pollution-management levels: alter human activity, control release, or clean-up.
2. Give the benefit, with data if you have it.
3. Give at least two limits: cost, enforcement, displacement, timescale, who pays.
4. Name the perspectives of at least two stakeholders.
5. Reach a judgement that answers the question.

DTM at a glance

Stage	Birth rate	Death rate	Why it changes
1	High	High	Little healthcare, poor food security, disease
2	High	Falls fast	Clean water, sanitation, vaccination, better food supply
3	Falls	Low	Contraception, education of women, urban living, fewer infant deaths
4	Low	Low	Choice of small families, career priorities, cost of children
5 (proposed)	Below CDR	Low or rising	Ageing population, very low fertility

Limits of the model: based on European history; assumes birth rates will follow death rates down; ignores migration; no timescale; stage 5 was added later and is not explained by the original model.

Urban system in one box

INPUTS                STORAGES               OUTPUTS
food, water           buildings              solid waste, sewage
energy (fuel, power)  infrastructure         air pollutants, CO₂
building materials    people                 waste heat
goods, migrants       stocks of goods        goods, services, emigrants

A city is an open system that relies on land far beyond its boundary, so its ecological footprint is much larger than its area.

Push factors (rural): few jobs, low incomes, poor services, drought, land degradation, farm mechanisation. Pull factors (urban): jobs, wages, education, healthcare, perceived opportunity.

Sustainable planning toolkit: compact mixed-use design; brownfield before greenfield; public transport and cycling; green roofs, parks and street trees; energy- and water-efficient buildings; waste separation and recycling.

Urban heat island causes: dark surfaces absorb and store heat; waste heat from vehicles, buildings and air conditioning; less vegetation, so less evapotranspiration; tall buildings trap heat and reduce wind.

Managing urban air pollution

Level	Strategy	Main limitation
Alter human activity	Public transport, cycling, compact planning, renewable electricity	Slow; needs investment and public support
Alter human activity	Congestion charge	Can fall hardest on lower-income drivers
Control release	Catalytic converters, particulate filters	Only work if fitted and maintained; do not cut vehicle numbers
Control release	Low-emission zone, emission standards	Traffic may divert to other roads; needs enforcement
Control release	Scrubbers on power stations and factories	High cost for the operator
Clean-up	Street trees, green walls	Small effect on city-wide concentrations
Clean-up	Air quality alerts	Protects people but does not reduce pollution

Must-know distinctions

• Urbanisation vs urban growth: urbanisation is a rising share of people in towns; urban growth is a rising number. Both can happen together.
• Primary vs secondary pollutant: NO₂ from exhausts is primary; ozone formed from it is secondary.
• Tropospheric vs stratospheric ozone: ground-level ozone is a harmful pollutant; stratospheric ozone absorbs UV and protects life.
• CBR vs TFR: CBR depends on the age structure; TFR measures births per woman and is better for comparing countries.
• Natural increase vs population growth: population growth also includes net migration.
• Pro-natalist vs anti-natalist: policies to raise vs lower the birth rate.
• Malthus vs Boserup: population limited by resources vs population pressure driving innovation.

Quick self-test

1. Define total fertility rate.
2. Why is 2.1, not 2.0, taken as replacement level?
3. A country has CBR 24 per 1,000 and CDR 9 per 1,000. Find the NIR and the doubling time.
4. A population is 18% aged 0–14, 60% aged 15–64 and 22% aged 65+. Find the dependency ratio.
5. Give two reasons the death rate falls in stage 2 of the DTM.
6. State two limitations of the DTM.
7. Give one input and one output of an urban system that are forms of energy.
8. Is ground-level ozone a primary or a secondary pollutant? Explain.
9. A country of 18 million people has 6.3 million urban residents. What percentage is urban?
10. After a city bans coal heating, mean winter PM2.5 falls from 45 to 27 µg/m³. Find the percentage change.
11. Explain why a city in a valley may suffer worse smog episodes.
12. Give one example each of a pro-natalist and an anti-natalist measure.

Answers

1. The average number of children a woman would have over her lifetime at current age-specific birth rates.
2. Some children die before they reach child-bearing age, so slightly more than two births per woman are needed to replace both parents.
3. NIR = (24 − 9) ÷ 10 = 1.5%; doubling time ≈ 70 ÷ 1.5 = 47 years (46.7).
4. (18 + 22) ÷ 60 × 100 = 66.7 dependants per 100 people of working age.
5. Any two: clean water; better sanitation; vaccination and healthcare; more reliable food supply.
6. Any two: based on Europe’s history; assumes birth rates fall after death rates; ignores migration; no timescale.
7. Input: fuel or electricity. Output: waste heat.
8. Secondary: it forms in the air when sunlight splits NO₂ and the free oxygen atom joins O₂.
9. 6.3 ÷ 18 × 100 = 35%.
10. (27 − 45) ÷ 45 × 100 = −40% (a 40% fall).
11. Cold air drains into the valley at night and warm air sits above it, forming an inversion; the hills also block wind, so pollutants are not dispersed.
12. Pro-natalist: child benefit or paid parental leave. Anti-natalist: free contraception or family planning services.

Where marks are usually lost

• Writing NIR as “15” instead of “1.5%” because the ÷ 10 step was missed.
• Quoting a doubling time without stating the assumption of a constant growth rate and no migration.
• Describing a pyramid with words like “lots of young people” and no figures read from the graph.
• Calling ozone or PANs primary pollutants.
• Explaining an inversion as “hot air trapping pollution” without saying that warm air lies above cool air.
• Explaining smog without sunlight, the trigger for the reaction.
• Listing urban planning strategies without saying which pollution-management level they act on.
• One-sided evaluation: benefits only, with no cost, enforcement issue or stakeholder view.
• Treating urbanisation and urban growth as the same thing.
• Ending a “to what extent” answer without a clear judgement.

Official syllabus

International Baccalaureate Organization, Diploma Programme Subject Brief, Environmental systems and societies, first assessment 2026. The brief lists subtopics for Topic 1 only; the subtopic numbering on this page follows the printable ESS checklist.