| Micrograms (μg) | Tonnes (t) |
|---|---|
| 1 Microgram | 1 × 10-12 t |
| 2 Micrograms | 2 × 10-12 t |
| 3 Micrograms | 3 × 10-12 t |
| 4 Micrograms | 4 × 10-12 t |
| 5 Micrograms | 5 × 10-12 t |
| 10 Micrograms | 1 × 10-11 t |
| 20 Micrograms | 2 × 10-11 t |
| 25 Micrograms | 2.5 × 10-11 t |
| 50 Micrograms | 5 × 10-11 t |
| 100 Micrograms | 1 × 10-10 t |
| Reference | Micrograms (μg) | Tonnes (t) |
|---|---|---|
| A sheet of A4 paper (80 gsm) | 5000000 μg | 0.000005 t |
| One litre of water | 1 × 109 μg | 0.001 t |
| Average adult human | 7 × 1010 μg | 0.07 t |
| A small car | 1.4 × 1012 μg | 1.4 t |
The microgram is a unit of mass equal to one millionth of a gram, or one thousandth of a milligram. Its official symbol is µg, written with the Greek letter mu.
In medicine that symbol is a known hazard. Handwritten, µg is easily read as mg, a thousandfold error, and the mistake has killed patients. Safety bodies including the Institute for Safe Medication Practices therefore recommend writing mcg instead on prescriptions and labels, and most hospital systems now enforce it. The abbreviation is not SI and would be wrong in a scientific paper, but it is the safer choice on a drug chart.
Micronutrients live at this scale. Vitamin B12, folate, iodine, selenium, biotin and vitamin K are all dosed in micrograms, and daily requirements run from a few micrograms to a few hundred. Vitamin D is given either in micrograms or in international units, with one microgram equal to forty IU, a dual convention that causes constant confusion on supplement labels. Potent drugs use the unit too: fentanyl, levothyroxine, digoxin and most hormonal contraceptives are dosed in micrograms rather than milligrams.
Air quality reporting has made the microgram familiar to the public. Fine particulate matter is measured in micrograms per cubic metre, and the World Health Organization guideline for annual PM2.5 exposure is five micrograms per cubic metre. Nitrogen dioxide, sulphur dioxide and ozone limits are written the same way, so the number quoted in a news report about pollution is almost always a count of micrograms in a cubic metre of air.
Toxicology works here because potency and hazard both scale with dose. Digoxin is prescribed in a few hundred micrograms a day and becomes dangerous not far above that, thyroid hormone is dispensed in 25-microgram steps, and the opioid fentanyl is active at doses of tens of micrograms, which is why illicitly mixed powders containing it are so often lethal. Analytical chemistry meets the unit from the other direction: modern instruments detect pesticides, metals and pharmaceuticals in water at concentrations of micrograms per litre, and regulatory limits are written at that level. A microgram is invisible, weighs less than a speck of dust, and yet is the natural unit for both the dose that heals and the dose that kills.
One microgram equals 0.001 milligrams, one millionth of a gram, and one thousand nanograms.
The tonne is a unit of mass equal to 1000 kilograms. Its symbol is t. It is not an SI unit but is accepted for use with the SI, and in most of the world it is simply the working unit for anything too heavy to state comfortably in kilograms.
The name causes trouble in English. A tonne, sometimes written metric ton, is 1000 kilograms. A short ton, the American ton, is 2000 pounds or about 907 kilograms. A long ton, the British imperial ton, is 2240 pounds or about 1016 kilograms. The three differ by up to twelve per cent, which is large enough to matter in shipping and commodity contracts, so international documents normally spell out which is meant rather than relying on the word alone.
Freight and industry are its home. Ship cargo, rail wagons, steel production, cement, grain harvests and vehicle gross weights are all quoted in tonnes. Bridge and crane load ratings use it, and so does the deadweight tonnage of a ship, although that particular term measures carrying capacity rather than the mass of the vessel itself.
Climate accounting has given the tonne a second public life. National and corporate emissions are reported in tonnes of carbon dioxide equivalent, and carbon markets price allowances per tonne. A single average petrol car emits roughly two tonnes of carbon dioxide a year, which is why the unit appears in policy documents far outside engineering.
Energy accounting borrows the word for a quantity that is not a mass at all. The tonne of oil equivalent is defined as the energy released by burning one tonne of crude oil, about 41.87 gigajoules, and national energy balances are published in millions of them. A tonne of coal equivalent serves the same purpose at a different value. These are conventions rather than measurements: no particular tonne of oil delivers exactly that figure, but fixing one lets gas, coal, nuclear output and renewables be added together on a single scale. Readers of energy statistics therefore meet the tonne twice, once as a mass of fuel and once as a package of energy.
One tonne equals 1000 kilograms, about 2204.62 pounds, or approximately 1.10231 short tons.