Powers of ten: from your hand to a germ

Hold up your hand. It is about 10 centimetres across. Now imagine zooming in so that everything looks ten times bigger, then ten times bigger again. Each zoom is one step. A zoom of ten times, over and over, is called a powers-of-ten journey.

Here are the stops on this journey. Sizes are typical values, because real things vary: a hair can be much thinner or thicker than the one here.

• Hand: about 10 cm
• Grain of salt: about 0.3 mm (300 µm)
• Human hair, thickness: about 70 µm
• Skin cell: about 30 µm
• Red blood cell: about 7 µm
• E. coli bacterium: about 2 µm long and 0.5 µm wide
• Influenza virus particle: about 100 nm
• A typical protein: about 5 nm
• A water molecule: about 0.3 nm

Count the steps from your hand to a virus. 10 cm is 10⁻¹ m and 100 nm is 10⁻⁷ m, so you need six zooms of ten times each. That is a factor of 1,000,000 (10⁶). If a virus particle were the size of your hand, your hand would stretch about 100 km, the distance of a long road trip.

People say germ in everyday speech for any tiny organism that can make you ill. Scientists say microbe or microorganism for any living thing too small to see without a microscope. Most microbes are harmless. Many are helpful: they help you digest food, turn milk into yoghurt and recycle dead leaves. Only a small fraction can cause disease.