Intermediate Physics for Medicine and Biology: The Dobson Unit

Friday, October 6, 2023

The Dobson Unit

Figure 14.28 from Intermediate Physics for Medicine and Biology, showing the spectral dose rate weighted for ability to damage DNA.

In Chapter 14 of Intermediate Physics for Medicine and Biology, Russ Hobbie and I discuss the risk of DNA damage—and therefore cancer—caused by ultraviolet light from the sun. Figure 14.28 in IPMB presents the results of a calculation of UV dose rate, weighted for DNA damage. The caption of the figure states “the calculation assumes clear skies and an ozone layer of 300 Dobson units (1 DU = 2.69 × 10²⁰ molecule m^-2).”

The Dobson Unit, what’s that?

Rather than explaining it myself, let me quote the NASA website about ozone.

What is a Dobson Unit?

The Dobson Unit is the most common unit for measuring ozone concentration. One Dobson Unit is the number of molecules of ozone [O₃] that would be required to create a layer of pure ozone 0.01 millimeters thick at a temperature of 0 degrees Celsius and a pressure of 1 atmosphere (the air pressure at the surface of the Earth). Expressed another way, a column of air with an ozone concentration of 1 Dobson Unit would contain about 2.69 × 10¹⁶ozone molecules for every square centimeter of area at the base of the column. Over the Earth’s surface, the ozone layer’s average thickness is about 300 Dobson Units or a layer that is 3 millimeters thick.

The Dobson Unit was named after British physicist and meteorologist Gordon Miller Bourne Dobson (1889 –1976) who did early research on ozone in the atmosphere.

Worried about climate change? The ozone story may provide some hope. When man-made chemicals such as chlorofluorocarbons, for example freon, are released into the atmosphere, they damage the ozone layer, allowing larger amounts of ultraviolet radiation to reach the earth’s surface. In the 1970s, an ozone hole developed each year over the south pole. In 1987, countries from all over the world united to pass the Montreal Protocol, which banned many ozone depleting substances. Since that time, the ozone hole has been getting smaller. This represents a success story demonstrating how international cooperation can address critical environmental hazards. Now, we need to do the same thing for greenhouse gases to combat climate change.