sky with clouds and sun depicting sheild of ozone layer

The ozone layer is miles above us but protects us from most of the sun's harmful ultraviolet rays.


Introduction to How the Ozone Layer Works

­If you've ever gotten a nasty sunburn, yo­u've experienced the singeing effects of ultraviolet radiation from the sun. In fact, you probably made a personal vow to forever apply (and reapply) sunscreen on sunny days. Luckily, the earth shades us from the vast majority of intense ultraviolet light with its own sunscreen -- the ozone layer. Without the ozone layer, we wouldn't just sunburn, there's a chance we'd go extinct. The sheer intensity of unadulterated ultraviolet sunlight would threaten most species that live on the surface of the earth.


The ozone layer gets its name from ozone gas, an allotrope, or form, of the element oxygen. Ozone gas has become so synonymous with the ozone layer that people now refer to the layer as "the ozone." However, ozone gas doesn't just coat our earth's stratosphere. It can be found on the earth's surface as well -- used for such things as bleaching, sterilizing water, and removing unpleasant smells from products.

Despite its name, the ozone layer isn't just ozone gas. Oxygen gas, another allotrope of oxygen, is abundant in the ozone layer, as well. Oxygen gas is essential for the creation of ozone gas, and it absorbs ultraviolet light, preventing that light from reaching the earth's surface. The ozone layer forms naturally in the stratosphere, where ozone and oxygen gas are always converting into each other -- continually "reapplying" the earth's sunscreen.

Scientists study the ozone layer to grasp its patterns of change and to determine how humans affect this change. If the ozone is suffering, how are we to blame? First, let's investigate what this layer really is.


oxygen-ozone cycle

When UV light hits oxygen gas, it breaks it down to two oxygen atoms. Then, when an oxygen atom meets oxygen gas, it forms ozone gas.


How the Ozone Layer Forms and Protects

Most ecosystems rely on the ozone to protect them from harmful ultraviolet (UV) light. If you know much about the light spectrum, you'll remember that the varying wavelengths of light determine the color or kind of light. Ultraviolet light falls outside of the range of light that's visible to the human eye, much like microwaves, X-ray and radio waves.

When it comes to UV light, what we don't know (or don't see) can hurt us. UV light from the sun's rays burns our skin and freckles our noses when we're outside on a sunny day. But skin blemishes are the least of our worries. Exposure to UV light can lead to skin cancer and cataracts, and can damage the body's immune system [source: EPA].

Thankfully, the ozone layer protects us from most of the sun's harmful UV rays. Ninety percent of the atmospheric ozone is in the earth's stratosphere -- the altitude starting at six to 11 miles (9.6 to 17.7 kilometers) above the earth and extending to about 30 miles (48.3 kilometers) above the earth [source: Fahey]. The stratosphere provides a natural setting conducive to the formation of the ozone, where gas forms a protective layer that completely envelops the earth.

Ozone gas forms in the stratosphere when UV sunlight hits oxygen gas in what is known as the ozone-oxygen cycle:

  • The first stage of this cycle occurs when short-wavelength UV light from the sun hits a molecule of oxygen gas. The light has so much energy that it breaks the oxygen bond holding the atoms together, thus creating two oxygen atoms. Through this process, the oxygen essentially absorbs the short-wavelength UV light, but this still leaves a significant amount of UV light with longer wavelengths, which is where ozone comes in.
  • In the second stage, each of the two remaining oxygen atoms will then latch onto two oxygen gas molecules, creating two separate ozone molecules [source: Fahey].
  • Short-wavelength UV light has enough energy to break apart ozone molecules (which are more volatile and easier to separate than oxygen molecules). Thus, in the third stage of the cycle, the ozone gas then breaks into one oxygen gas molecule and an oxygen atom, hence absorbing much of the remaining UV light.

If you're wondering why these processes "absorb" UV light, it is because they create exothermic reactions, meaning they release heat. Essentially, oxygen and ozone convert UV light to heat. Together, ozone and oxygen gas are effective at absorbing about 98 percent of the harmful UV light [source: Sparling].

On the next page, we'll discuss the different methods and instruments scientists use to measure ozone levels in the ozone layer.

image of ozone measurements

We can get measurements of the ozone layer from instruments on satellites in space. One of the TOMS instruments gave scientists data to create this image depicting ozone levels.

Space Frontiers/Stringer/Hulton Archive/Getty Images

How Scientists Measure the Ozone

Scientists are able to study the amount of ozone in a given vertical column of atmosphere by using various instruments. One such instrument is an ozonesonde ("sonde" comes from Old English, meaning messenger), which includes a balloon that carries the instrument up more than 21 miles (33.8 kilometers) high to the stratosphere [source: NOAA]. In the stratosphere, it sucks in and holds air to test the amount of ozone gas using an electrochemical concentration cell (ECC). The ECC uses potassium iodide, which reacts with ozone to create an electrical current, to measure the amount of ozone present [source: NOAA]. Although the balloon can burst when it reaches too high of an altitude, the device includes a parachute to reduce damage when it lands.

In addition to these ozonesondes, aircraft that can fly especially high can also reach the lower stratosphere to measure the ozone in the air. Also, UV detectors on the ground measure how much UV light has penetrated the ozone layer to reach the surface of the earth, which gives us clues as to how much ozone is in the atmosphere. Both ground stations and planes can use lasers to detect ozone, as well.

Other instruments scientists use include instruments on satellites, such as the TOMS (Total Ozone Mapping Spectrometer). The TOMS instrument determines the amount of ozone present in the ozone layer by reading backscattered UV light, which is the UV light the earth emits back into space [source: NASA]. Although the TOMS program closed in 2007 after the latest TOMS instrument started failing to transmit information, the program played an important role in revealing the state of the ozone for 30 years [source: Spector]. Meanwhile, different types of instruments have been commissioned to measure the ozone, such as the Ozone Monitoring Instrument (OMI) on the Aura satellite, which also measures backscattered UV light.

Because ozone gas is present among other atmospheric gases in various densities and in various altitudes, determining the "thickness" of the ozone depends on how you look at it. If you brought all the atmospheric ozone gas down to one layer, it would only be about a quarter of an inch thick (0.6 centimeters) [source: Fahey]. But, in reality, ozone gas spreads out in the stratosphere and works with oxygen to protect us. By this measure, the scope of the ozone layer is about 25 miles thick (40 kilometers) [source: Encyclopedia Britannica].

When scientists determine the amount of total ozone, they measure the amount of ozone gas in a column of air. To measure ozone, they use Dobson units (DU), named after a pioneer in ozone research, G.M.B. Dobson. One Dobson unit indicates 0.01 millimeter thickness of ozone gas in a column [source: NASA].

Using these techniques, scientists have been able to determine how much ozone gas is present in the stratosphere. This has led to some disquieting discoveries, which we'll talk about next.

aerosol can spraying cfcs

Scientists believe CFCs, chemicals released from aerosol cans, AC units and refrigerators, are largely to blame for ozone depletion.

Caspar Benson/Getty Images

It Happens Every Spring: How a Hole Forms in the Ozone

In the 1970s, scientists discovered that chemicals known as CFCs, which stands for chlorofluorocarbons, can reach the stratosphere and destroy ozone gas. This was a weighty discovery with worldwide implications because most of us rely on several products and appliances that, when manufactured or used, release CFCs into the atmosphere. Aerosol spray cans, Styrofoam, air conditioner units and refrigerators are a few items that make the list. The scientists began claiming that CFCs contribute to the formation of "holes" in the ozone.

How could this happen? And how could CFCs be partly responsible? Chemists Mario Molina and Sherwood Rowland were awarded the Nobel Prize in 1995 for their theories and research that explained how this might work. Scientists knew that chlorine and bromine are both substances that can destroy ozone. It turns out that some natural and man-made chemical compounds containing chlorine and bromine are able to rise up to the stratosphere where the conditions allow them to react with and destroy ozone. The earth's natural production of these substances accounts for 17 percent of the chlorine and 30 percent of the bromine in the stratosphere [source: Fahey].

chlorine's effect in the ozone layer

When chlorine encounters ozone, chlorine monoxide and an oxygen molecule form (destroying the ozone). When chlorine monoxide encounters an oxygen atom, the chlorine is released to wreak havoc on more ozone.


Molina and Sherwood explained that CFCs, which are man-made, gradually rise up into the ozone layer, where ultraviolet light breaks the compounds apart, which releases chlorine [source: Nobel Foundation]. A chlorine atom can steal an oxygen atom from an ozone molecule, creating oxygen gas and chlorine monoxide (ClO), which effectively destroys the ozone molecule [source: Chemical Heritage]. But the chlorine atom isn't done yet; a chlorine atom can break from its oxygen atom and wreak havoc on as many as 10,000 more ozone molecules [source: UCS]. From their findings, the chemists projected that after years of unrestrained CFC production, the ozone would deplete significantly.

hole in ozone layer

For several decades, scientists have been tracking the hole in the ozone layer that forms over the Antarctic every spring.

Getty Images

When scientists and popular media refer to a "hole" in the ozone, what they really mean is an area with low DU, or where the vertical column of ozone layer (which, as we found, spans about 25 vertical miles) includes little ozone gas compared to the other areas. In one sense, the ozone "hole" can be understood as a "thin" area of the layer because if we collected all the ozone in that vertical area, it would be thinner than other places over the earth. Specifically, scientists are worried that production of CFCs will lead to a "hole" over Antarctica.

Every year, the ozone levels over Antarctica sink drastically during the Southern Hemisphere's spring. Scientists believe this began happening in the late 1970s as a result of CFCs. The hole forms in the Antarctic because cold air becomes trapped there as a result of the polar vortex -- strong, circulating winds. The cold temperatures allow the formation of polar stratospheric clouds (PSCs), or ice clouds. These PSCs are conducive to the breakdown of chlorine-containing compounds, which are there because of our production of CFCs. This makes the area especially susceptible to ozone depletion. When sun hits PSCs in early spring, large amounts of chlorine monoxide form from the substances that contain chlorine. Fortunately, by early summer, ozone from other areas comes in to help fill this hole [source: Fahey]. But because of our CFC production, the hole returns each year. For more information on this phenomenon, read "Can we plug the hole in the ozone layer?"

Next, we'll take a closer look at ozone depletion and what it means for us on earth. Are there any efforts that could prevent it from getting worse?

ozone-friendly materials without cfcs

The Montreal Protocol was an effort to encourage industries to switch to "ozone friendly" products.

Stockbyte/Getty Images

The Danger and Prevention of Ozone Layer Depletion

When you think about ultraviolet light, do you think black lights? Black lights use harmless ultraviolet light, but the ultraviolet light that the ozone layer absorbs would be extremely dangerous if it came in contact with our skin. There are many different types of UV light, and their classification depends on their wavelengths. UV light comes in the following forms:

  • UVA (between 320 and 400 nanometers): Ozone does not absorb UVA light.
  • UVB (between 280 and 320 nanometers): This can be harmful to skin, and, thankfully, the ozone protects us from much of it. But even a healthy, "thick" ozone layer doesn't prevent all UVB from reaching the earth.
  • UVC (less than 280 nanometers): The ozone layer prevents all UVC from reaching the earth's surface. That's a good thing because UVC would be very harmful to our ecosystems.

The UV intensity on the surface of the earth fluctuates from day to day and depends on where you live. These days it's easy to check on the local UV index, which tells you how intense the UV radiation is in your area. In the U.S., the UV index is calculated from various factors, including ozone measurements and local cloud-coverage predictions [source: EPA]. Meteorologists measure UV radiation in terms of its energy in an area over time, such as milliwatts per square centimeter per second. They express this in weather reports in numbers and colors.

  • Low (Green) = 0-2
  • Moderate (Yellow) = 3-5
  • High (Orange) = 6-7
  • Very High (Red) = 8-10
  • Extreme (Purple) = 11 and higher

Even with the ozone layer, UVA and especially UVB light can penetrate it to cause these health problems for humans:

  • Skin cancer: The American Cancer Society cites UV radiation as the primary cause of skin cancer [source: ACS]. When UV light damages DNA , it impairs its ability to control skin growth.
  • Other skin problems: Overexposure to UV light can lead to skin effects that resemble premature aging, as well as skin lesions known as Actinic keratoses.
  • Immune suppression: Evidence suggests that the body's immune system will begin to weaken as a result of too much UV light.
  • Eye problems: Cataracts can develop from UV exposure, which essentially clouds sight and can lead to blindness.

­Given that these risks exist even when the ozone layer absorbs about 98 percent of UV light, a future with an even thinner ozone layer is scary indeed. In the 1980s, this frightening prospect motivated the scientific community and policy makers of the world to address the issue of CFC production, which resulted in the 1987 Montreal Protocol. Policy makers from different industrialized countries signed this treaty as a pledge to reduce their CFC outputs. International Day for the Preservation of the Ozone Layer is celebrated each year on the anniversary date of the treaty, September 16th.

Efforts to find a replacement for CFCs have been somewhat successful. For instance, HCFCs (Hydrochlorofluorocarbons) can also potentially deplete ozone gas in the stratosphere, but not nearly to the extent of CFCs [source: UCS].

The efforts seem to be working. Reports show that by 2049, the ozone will recover to 1979 thickness, which is when scientists believe the hole began to form [source: Reuters]. Many scientists believe the Montreal Protocol is responsible for this joyous recovery.

To learn much more about the ozone and other related subjects, explore the links on the next page.

Lots More Information

Related ArticlesMore Great LinksSources
  • ACS. "UV Radiation & Cancer." American Cancer Society. 2006. (April 17, 2008)
  • Chemical Heritage."Making and Destroying Ozone." Chemical Heritage. 2001. (April 17, 2008)
  • EPA. "Health Effects of UV Radiation." Environmental Protection Agency. (April 17, 2008)
  • EPA. "How is the UV Index Calculated?" Environmental Protection Agency. Jan. 3, 2008 (April 21, 2008)
  • EPA. "Ozone Depletion Glossary." Environmental Protection Agency. Mar. 14, 2008. (April 17, 2008)
  • Fahey, D.W. "Scientific Assessment of Ozone Depletion: Twenty Questions About the Ozone Layer: 2006 Update." World Meteorological Organization, March 2007. (April 15, 2008)
  • NASA. "TOMS - EP." National Aeronautics Space Administration. Mar. 6, 2008. (April 17, 2008)
  • NASA. "What is a Dobson Unit?" National Aeronautics Space Administration Feb. 1, 2008. (April 17, 2008)
  • NOAA. "Ozonesonde." National Oceanic and Atmosphereic Administration. Mar. 20, 2008. (April 17, 2008)
  • "ozonosphere." Encyclopaedia Britannica. (April 17, 2008)
  • Reuters. "Ozone Layer Healing, but More Slowly Than Hoped." The Washington Post. Aug. 19, 2006.
  • Sparling, Brien. "Ultraviolet Radiation." National Aeronautics Space Administration. Mar. 30, 2001. (April 17, 2008)
  • Spector, Laura.  "R.I.P. TOMS: NASA Ozone Instrument Laid to Rest After Three Decades." National Aeronautics Space Administration. Aug. 15, 2007. (April 17, 2008)
  • The Nobel Foundation. "The Nobel Prize in Chemistry 1995: Press Release." The Nobel Foundation. Oct. 11, 1995.
  • UCS. "Global Warming: Frequently Asked Questions about Ozone Deletion and the Ozone Hole." Union of Concerned Scientists. Aug. 10, 2005. (April 17, 2008)­