A U.S. Environmental Protection Agency fact sheet on contrails explains that contrails, even long-lasting ones, are simply trails of condensation and are not harmful. However, conspiracy theorists have become suspicious that the contrails expelled from jets today are thicker and linger longer than they did in the past. So, while they accept that contrails are a natural byproduct of jet engines, their suspicion is that the government has since used that excuse to put other substances in them, creating chemtrails. In addition, conspiracy theorists latch on to anecdotal evidence that connect epidemics of flulike symptoms to instances of contrails. Those who believe that there's something fishy about contrails have come up with several theories to explain what the government could be covering up.
One of the more extreme theories says that the government is intentionally spraying people with harmful substances in order to experiment with the effects -- or even to "weed out" the sick and elderly. However, many reputable scientists dismiss these theories on the grounds that such experiments would be of no real use. The "chemtrails" would be released so high in the atmosphere that unpredictable winds would move them around significantly, making such experiments worthless and unreliable [source: Hodapp]. Others speculate that the government is dumping barium salt aerosol on the land in order to assist in radar mapping for defense purposes [source: Knight].
Still others believe that the government could be experimenting with weather manipulation for defense purposes. This actually isn't as much of a stretch as it may seem. As long ago as the 1950s, the British were able to successfully "seed" clouds with salt, dry ice and silver iodide to make rain (see the sidebar on this page). It would seem that weather manipulation, then, is a very possible and effective military defense tactic. Conspiracy theorists believe it might have connections to HAARP, the High Frequency Active Auroral Research Program, which studies the atmosphere to advance communication and navigation systems.
Another popular theory is that chemtrails are well-intentioned attempts by the government to combat global warming or the depletion of the ozone layer by spraying particles to reflect the sun's radiation. However, if this is true, it's ironic that (non-conspiracy theorist) environmentalists blame contrails for polluting the skies. They say that jet traffic has become so bad that the sheer cloud cover from contrails, which can be seen from space, has been negatively affecting the environment, possibly contributing to global warming.
So, whether you consider the conspiracy hype healthy skepticism or paranoia, contrails (along with the fuel consumption and energy use of the air traffic industry that cause them) are certainly an area of concern for the environment.