Evil Chlorine: The Science of DDT, PCBs, and CFCs

A lot of chlorine compounds (molecules with at least one chlorine atom) are considered “evil” or toxic, and they have subsequently been banned.  These include polychlorinated biphenyls, DDT (dichloro-diphenyl trichloroethane), and the very well known chlorofluorocarbons (CFCs).  But what makes these compounds so dangerous?  Here’s a little bit about each of them:

a) Chlorofluorocarbons (CFCs): CFCs originally were manufactured, not found in nature, to make refrigeration more efficient and less expensive.  Refrigeration works by liquid evaporating by absorbing heat from its surroundings, then the vapor condenses, returning to liquid state, then re-evaporates.  This process causes cooling and keeps the inside of refrigerators warm.  The ideal refrigerant has to vaporize within the proper temperature range and absorb lots of heat while doing so.  This means it has to be a very stable compound so it absorbs a lot of heat before it evaporates, instead of very quickly evaporating.  The discovery of CFCs revolutionized the refrigeration business because it was stable, nontoxic, nonflammable, almost odorless, and inexpensive – it functionally made the refrigerator a standard home appliance and made air conditioning an industry.

But…CFCs are so stable, they don’t break down by ordinary reactions.  These compounds can stay in the atmosphere from 40-150 years.  When factories and homes spew out CFCs, they get released into the lower atmosphere and drift around for years, eventually reaching the stratosphere.  Solar radiation then ruptures the CFCs into individual atoms or smaller molecules, which can be extremely damaging to our ozone layer.

Why?  Well, normally solar radiation breaks up O2 (what we breathe) into just a single oxygen atom, which can then react with O2 to make O3 (ozone).  The reverse reaction occurs as well, creating an equilibrium where the same amount of ozone being created is being destroyed and we have a constant amount of ozone and we are all happy.

Then chlorine comes in.  Chlorine reacts with ozone to make a compound ClO and O2.  It then reacts with another ozone molecule to make Cl and O2, functionally taking ozone and breaking it down to stable compounds that won’t create more ozone.  Even worse, because clorine is stable it stays in the atmosphere for decades and one single chlorine atom can destroy 100,000 ozone molecules before being deactivated.  And, scientists have found that for every 1% ozone depletion, 2% more UV radiation penetrates the atmosphere.  This was a huge problem before CFCs were banned in many countries (though not all).

File:Area of the ozone hole.jpg

The hole in our ozone before CFCs were banned in some countries.  Found at: http://commons.wikimedia.org/wiki/File:Area_of_the_ozone_hole.jpg

Now, of course, we use other compounds for refrigeration and air conditioning, a list of which can be found here: http://www.epa.gov/ozone/snap/refrigerants/lists/index.html.

b) Polychlorinated Biphenyls (PCBs): This is another extremely stable molecule used in electrical insulators and coolants in reactors, capacitors, and transformers.  Then, people started hearing about health problems among workers at PCB plants, including a skin condition called chloracne, which is among the first symptoms of PCB poisoning, followed by damage to immune, nervous, endocrine, and reproductive systems.

So what is a PCB?  It is a manufactured molecule in which all the hydrogen atoms in a biphenyl molecule have been substituted for chlorine atoms, which makes this molecule incredibly stable and persists for a long time in the environment.  The problem is that this stability causes biomagnification.  Biomagnification is a phenomenon where the concentrations of a toxin increase as you go up trophic levels (or the food chain); so if a small rat eats something with PCB in it, then a snake eats the rat, then a hawk eats the snake, the hawk has a much higher concentration of PCB than the rat.  This PCB can build up in fat cells and cause many health problems.

c) DDT: The same biomagnification problem is present in DDT.  DDT is a very stable pesticide molecule that was used in World War II as a delousing powder to stop typhus and kill disease carrying mosquitoes.  It was actually put in an aerosol can (which uses CFCs to work), which doubles the bad effect on the environment.

The big problem with DDT is that it also accumulates in animal tissues.  When this toxin reaches birds, the DDT actually inhibits an enzyme that supplies calcium to eggshells.  Then, birds lay eggs with really fragile shells that end up breaking before the offspring can hatch out.  This caused a significant decline in eagle, hawk, and falcon populations.

So, the common theme among all these chlorine compounds is that they are dangerous mostly due to their incredible stability; the bonds that the chlorine atoms form are very strong and do not break down or react easily.

But, these chlorine compounds also did wonders for the world – they revolutionized the idea of home refrigeration and air conditioning and opened up a lot of trade opportunities.  So, are these chlorine compounds good or bad?  A little of both: they’ve done a lot of good, but for now they are better off being left out of our environment.

Works Cited





Chemistry of PCBs and PBBs – by I. Pomerantz, J. Burke. D. Firestone, J. McKinney, J. Roach, W. Trotter – Environmental Health Perspectives, Vol. 24, June 1978, pp. 133-146

Chlorofluorocarbons and the Depletion of Stratospheric Ozone – by F. Sherwood Rowland, American Scientist Vol. 77 No. 1 (January-February 1989), pp. 36-45

Napoleon’s Buttons – by Penny Le Couteur, 2004


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