That is actually a great question that honestly, I do not have the answer to. However, we do release various poisons in the air through various vehicles, and industry (factories, machines, etc.). Of course, there is an economic trade-off from this; on the other hand, chemtrails could be sprayed for a variety of reasons. These could include: possible weather manipulation, biological experimentation, or "include conspiracy theory here".

If you check out the first part of the "Zombie Apocalyps" video you can see how they carried out biological experimentation in New York City. In addition, there has been a long historical record of human experimentation on the public by the government and various corporations:

https://en.wikipedia.org/wiki/Unethical_human_experimentation_in_the_United_States

http://www.nbcnews.com/id/41811750/ns/health-health_care/t/ugly-past-us-human-experiments-uncovered/