Many 'tree farmers' spray trees to increase the shelf-life. We know bugs and fungi will attack organic matter such as the Christmas trees once there is the opportunity to. There are pesticides and herbicides used throughout the life of the tree and these are all known to have residual effects that can last many years.
Surveys carried out by Dr. Steve Toth, Pesticide Impact Specialist with North Carolina State University found the use of many pesticides and herbicides such as:
These help greatly to increase the shelf lives of the trees but we have some idea as the the kind of effects these chemicals can have especially after long-term exposure. It can be argued that the kind of exposure we undergo from the short period of owning the these trees would not necessarily be cause for concern. However, it is my guess and many other people's that anything designed to kill bugs and weeds is usually toxic to humans. Therefore there ought to be some concern there.
You can always ask who you purchase the tree from about their chemical usage and this sort of information should be readily available to the customer. If you want to get trees that are not exposed to such chemicals, you can purchase certified organic trees. Organic farming reduces and in most cases eliminates the use of such chemicals. These methods are a bit more expensive but it helps in reducing or limiting our exposure to chemicals.