Why a Banana Peel Vanishes in Weeks but a Plastic Bottle Stays for Generations

A heap of vegetable peels behind a house is gone within a few weeks, turned into soil by bacteria and fungi. A plastic bag dropped beside it is still there years later. Both are waste, but only one can be recycled by nature.

This part covers pollution of air, water and soil, three special kinds of pollution, the two types of waste, and why lasting wastes such as plastics and DDT are so harmful.

Pollution is any undesirable change in air, water or soil that harms living things, and it is caused by substances called pollutants released mainly by vehicles, industries, farming and household waste.

Air pollution

- Sources: vehicles, factories, thermal power plants, burning of crop stubble and garbage, construction dust
- Pollutants: carbon monoxide, ${\text{SO}}_2$, oxides of nitrogen, smoke, dust and other suspended particles

Water pollution

- Sources: untreated sewage, industrial waste, fertiliser and pesticide run-off from fields, oil spills, hot water from power plants
- Pollutants: disease-causing germs, heavy metals such as mercury and lead, nitrates and phosphates, oil

Soil pollution

- Sources: excess fertilisers and pesticides, industrial waste, dumped garbage and plastics, mining
- Pollutants: toxic chemicals, heavy metals, non-biodegradable waste

An everyday example. Smoke from burning crop stubble after harvest adds to the thick winter haze over many north Indian cities.

The boundary case. A substance can be useful in one place and a pollutant in another — fertiliser feeds a crop, but the same nitrates washed into a lake cause harm.

Biomedical waste is discarded material from hospitals and laboratories, radiation pollution comes from X-rays and radioactive substances, and noise pollution is unwanted loud sound; noise can be reduced by controlling its sources and blocking its spread.

Biomedical waste

- Sources: hospitals, clinics, laboratories
- Examples: used syringes and needles, bandages, blood, body tissues, expired medicines
- Danger: can spread infections such as hepatitis B and HIV, so it must be separated and safely destroyed

Radiation pollution

- Sources: overuse of medical X-rays, nuclear power plant leaks and waste, radioactive minerals
- Effects: burns, cancer and mutations

Noise pollution

- Measured in decibels (dB)
- Sources: vehicle horns, loudspeakers, firecrackers, aircraft, factories, construction
- Effects: hearing loss, stress, sleeplessness, high blood pressure

Ways to minimise noise:

- Silence zones near hospitals and schools
- Avoiding unnecessary honking and loudspeakers at night
- Silencers on vehicles and quieter machines
- Trees along roads, which absorb sound
- Ear protection for workers in noisy places

Worked example. On the decibel scale, every $10\text{ dB}$ rise means $10$ times more sound intensity. A $90\text{ dB}$ horn compared with $60\text{ dB}$ conversation:

$$90-60=30\text{ dB}10\times 10\times 10=1000\text{ times the intensity}$$

An everyday example. Signboards marking a silence zone outside hospitals ask drivers not to use their horns.

Biodegradable waste can be broken down by bacteria and fungi into harmless substances, while non-biodegradable waste cannot be broken down by them, or only extremely slowly.

Biodegradable waste

- Broken down by decomposers into simple substances that return to the soil
- Examples: vegetable and fruit peels, leftover food, paper, cotton cloth, wood, leaves, animal dung
- Management: composting turns it into manure

Non-biodegradable waste

- Not broken down by decomposers, so it stays in the environment for a very long time
- Examples: plastics, glass, metal cans, thermocol (Styrofoam), DDT and many other pesticides
- Management: reduce, reuse and recycle

An everyday example. Many municipalities ask households to separate waste into wet and dry bins, so the wet, biodegradable part can be composted and the dry part sent for recycling.

The boundary case. Biodegradable waste can still pollute in large amounts — sewage poured into a river uses up the oxygen that fish need as bacteria break it down.

These wastes last for a very long time: plastics, glass and thermocol pile up, block drains and harm animals, while DDT is stored in body fat and becomes more concentrated at each step of a food chain.

- Plastics — block drains and cause waterlogging; animals that swallow plastic bags can die; they break into tiny pieces that spread through soil and water; burning them releases toxic gases
- Glass — does not decompose, and broken pieces injure people and animals
- Thermocol (Styrofoam) — light and bulky, crumbles into small beads that animals swallow, and gives off harmful fumes when burnt
- DDT — a pesticide that is not broken down and not excreted; it is stored in fat

Biological magnification. DDT passes up a food chain:

$$\text{water}\to \text{plankton}\to \text{small fish}\to \text{large fish}\to \text{fish-eating bird}$$

Worked example. Suppose the concentration becomes $10$ times higher at each of the $4$ steps, starting from $0.001$ ppm in water:

$$0.001\times 10^4=10\text{ ppm in the bird}$$

That is **$10000$ times the level in the water. In birds, high DDT levels can make eggshells thin and easily broken.

An everyday example. Stray cattle eating from garbage heaps often swallow plastic bags, which collect in their stomachs.

The substance. The animals harmed most by DDT are often the top consumers**, which never came into contact with the spray.