Any substance with a pH lower than 7.

Acid Rain
Rain, snow, hail and fog which is acidic, as a result of pollution. Acid rain has a pH lower than 7.

Any substance with a pH higher than 7.

The layer of gases that surround and protects the Earth. It is about 700km thick.

Carbon Monoxide
A highly poisonous gas produced when fuel is burnt. It is mainly emitted from car exhausts.

Catalytic Converters
A filter fitted to car exhausts to help remove pollution.

CFCs / Chlorofluorocarbons
Chemicals used in products like aerosols and fridge.

Fossil Fuels
Fuels formed over a long time deep in the Earth from the remains of dead plants and animals. The main fossil fuels are coal, oil and natural gas.

Global Warming
Pollution has increased the Greenhouse Effect by putting more gases that trap heat into the atmosphere. This makes the Earth's temperature go up, and is called Global Warming.

Greenhouse Effect
The normal process by which heat is kept in the atmosphere. Without it, life could not survive on Earth. But if too much heat is trapped, temperatures could go up. This would then be dangerous to life.

Greenhouse Gas
Gases in the atmosphere, which can trap the heat escaping from the Earth, causing the temperature to go up.

Industrial Revolution
The rapid growth of industry which started in the late 18th century and was made possible by the harnessing of energy from fossil fuels such as coal.

Neither acidic nor alkaline, with a pH of 7.

Nitrogen Oxides
Polluting gases formed from nitrogen. They are produced when fossil fuels are burnt.

A gas that has no colour or smell. There is a lot of it in the atmosphere and vital for life.

A colourless gas, which forms the ozone layer in the atmosphere. This layer is very important for life on Earth as it stops harmful UV rays from the Sun reaching to Earth. Ozone is harmful for people's health, when nearer the ground.

Ozone Layer
A layer of ozone high up in the atmosphere, which shields us from the harmful rays of the Sun.

A tiny amount of solid or liquid carried in the air, for example soot, dust or fumes.

pH Scale
The pH scale is used to measure the amount of acid in a substance. The lower the number, the higher the amount of acid. Rain below pH 5 is said to be "acid rain".

Tiny microscopic creatures living in the sea. Some are animals - zooplankton, some are plants - phytoplankton.

A harmful substance emitted into the air, water or soil.

Energy that passes from a warm object to a cooler one, like energy from the Sun to the Earth - sunlight.

The countries of Sweden, Norway and Denmark, in the North of Europe.

Originally a mixture of smoke and fog. Now, air pollution produced by the reaction of hydrocarbons and nitrogen oxides in the presence of sunlight. It is mainly made up of ozone.

Sulphur Dioxide
A polluting gas formed from sulphur. It is produced when fossil fuels are burnt.

Ultra-Violet Rays
A type of radiation made by the Sun that is harmful to plants, animals and people. Sometimes called UV rays.