# 7.5 Balance of Oxygen and Carbon Dioxide · 7.6 Air Quality and Health

F.2 Integrated Science · Chapter 7 · Lesson 3

Why the two gases stay in balance, what extra carbon dioxide does, what pollutes the air, how the AQHI warns us, and why smoking harms.

## Key points

- **Balance**: respiration takes in oxygen and gives out carbon dioxide; photosynthesis takes in carbon dioxide and gives out oxygen. They keep oxygen (about 21 %) and carbon dioxide (about 0.04 %) in the air more or less **constant**.
- **Human activities** that raise carbon dioxide: **cutting down of trees**, **burning fossil fuels**, **industrial processes** and **more motor vehicles**.
- **Greenhouse effect ／ 溫室效應**: carbon dioxide (a **greenhouse gas**) traps heat from the Sun near the Earth's surface and keeps it warm. Too much carbon dioxide → an **excessive greenhouse effect** and hence **global warming**: higher average temperature, rising sea level, more extreme weather.
- **Ocean acidification ／ 海洋酸化**: more carbon dioxide dissolving makes sea water more acidic and threatens corals and shellfish. **Abnormal growth of plants**: some plants grow faster and may upset the balance of the ecosystem.
- **Low-carbon living ／ 低碳生活**: living in a way that gives out less carbon dioxide: plant trees, use less electricity, drive less, use alternative energy sources, reuse, recycle and reduce waste.
- **Air pollution**: solid particles and gases in the air at harmful concentrations: **poisonous gases** (carbon monoxide, ozone, sulphur dioxide, nitrogen oxides) and **suspended particulates ／ 懸浮粒子** (dust, smoke).
- **AQHI**: the **Environmental Protection Department** (EPD) measures pollutants at **air quality monitoring stations** and reports the **Air Quality Health Index** hourly: **1 to 10 and 10+**, **five** health risk categories; the higher, the poorer the air quality.
- **Smoking**: cigarette smoke contains **tar**, **nicotine** and **carbon monoxide**. Smokers cough more, get bronchitis more easily and breathe faster; the air-sac walls can break down (**emphysema ／ 肺氣腫**), reducing the surface area for gas exchange.
- **Pregnant women** who smoke have a higher chance of babies with a low birth weight. **Passive smoking** (breathing in smoke from smokers or lighted cigarettes) causes many of the same harmful effects.

## Must know

| Substance | Main sources | Harmful effects |
|---|---|---|
| Sulphur dioxide, nitrogen oxides, ozone | Power stations, factories, motor vehicles (ozone forms when sunlight acts on exhaust gases) | Irritate the eyes and throat, may cause difficulty in breathing; worsen asthma and bronchitis |
| Carbon monoxide (a colourless and odourless gas) | Motor vehicles, cigarette smoke | Lowers the oxygen content of the blood: headaches, dizziness, faster heartbeat, heart disease in the long term |
| Suspended particulates | Construction sites, factories, motor vehicles | Carbon particles irritate the lungs (worsen bronchitis and asthma); lead particles enter the blood and harm the brain |
| Tar (brown sticky liquid) | Cigarette smoke | Covers the air sacs and reduces the surface area for gas exchange; lung cancer; yellow teeth and fingernails |
| Nicotine (oily liquid) | Cigarette smoke | Raises blood pressure and heart rate, leading to heart disease in the long term; causes addiction to smoking |

The AQHI is worked out from four pollutants: ozone, nitrogen dioxide, sulphur dioxide and suspended particulates (not carbon dioxide). At 10+ the general public should minimize outdoor activities and time spent outdoors.

## Common mistakes

- ✗ "The greenhouse effect is harmful, so we should remove all the carbon dioxide from the air." ✓ The **natural** greenhouse effect keeps the Earth warm enough for life (without it the average surface temperature would be about −18 °C), and green plants need carbon dioxide for photosynthesis. Only the **extra** carbon dioxide from human activities, which enhances the greenhouse effect, is a problem.
- ✗ "The ozone hole enlarges because of global warming." ✓ The ozone hole is a different problem, caused by other gases (CFCs), not by carbon dioxide. It is not an effect of global warming.
- ✗ "Cutting down trees only means less oxygen is made." ✓ It adds carbon dioxide in two ways: fewer trees take it in by photosynthesis, and burning or rotting wood gives it out.
- ✗ "A high AQHI means good air quality; it is measured by the Hong Kong Observatory from carbon dioxide levels." ✓ The higher the AQHI, the **poorer** the air and the higher the short-term health risk. The EPD measures it at air quality monitoring stations from ozone, nitrogen dioxide, sulphur dioxide and suspended particulates.
- ✗ "Tar causes addiction; nicotine causes lung cancer." ✓ **Nicotine** causes addiction (and raises heart rate and blood pressure); **tar** causes lung cancer; **carbon monoxide** lowers the oxygen content of the blood.

## Quick check

1. Name the two processes that keep oxygen and carbon dioxide in the air constant, and state which gas each one takes in.
2. State two human activities that increase carbon dioxide in the atmosphere, and two effects of global warming.
3. Why is the greenhouse effect not harmful in itself? What turns it into a problem?
4. Which department reports the AQHI, what is its scale, and what should the general public do at 10+?
5. Match tar, nicotine and carbon monoxide to: addiction to smoking; lung cancer; less oxygen in the blood.

## Answers

1. Respiration takes in oxygen; photosynthesis takes in carbon dioxide.
2. Any two of: cutting down trees, burning fossil fuels, industrial processes, more motor vehicles. Effects: higher average temperature, rising sea level, melting polar ice, more extreme weather.
3. It traps heat near the Earth's surface and keeps it warm enough for life. Too much carbon dioxide from human activities makes it excessive, causing global warming.
4. The Environmental Protection Department (EPD); 1 to 10 and 10+ in five health risk categories; minimize outdoor activities and time spent outdoors.
5. Nicotine → addiction; tar → lung cancer; carbon monoxide → less oxygen in the blood.

## Did you know?

Venus is wrapped in a thick atmosphere of carbon dioxide. Its greenhouse effect makes the surface about 460 °C, hotter than Mercury, which is closer to the Sun.