I just read an article describing how researchers have identified that the epidemic of ADHD in America is actually related to a lack of dopamine.  Kids who have low levels of dopamine in the brain, which is what this article is discussing, have difficulty concentrating, thus the ADHD diagnosis. Yes, they can concentrate if you give them something particularly engaging, such as a computer game or something similar. But a lack of concentration and focus is their struggle.
Dopamine is actually produced in two primary locations in the body: the almond-sized hypothalamus, as a neurohormone, and the substantia nigra as a neurotransmitter. Dopamine cannot pass the blood brain barrier so the two are largely separate systems. Sadly, both sources of dopamine are under attack. As we cover in other links, the hypothalamus is hit hard by excitotoxins, creating a host of maladies.
Here I want to cover how dopamine in the substantia nigra is under attack from a different set of substances: pesticides. Many studies have shown that pesticides are correlated to damage of the substantia negra and are a significant factor in Parkinson's, the "Michael J. Fox" syndrome with tremors and muscle rigidity.
For example, one 2007 study found that Parkinson's was correlated in individuals who reported pesticide exposure and to certain pesticides.  Another Mayo Clinic study published shortly after found something similar: Parkinson's was significantly associated with pesticide/herbicide exposure.  And, as I mentioned in another link, one recent meta-analysis found that Vietnam vets exposed to Agent Orange and other herbicides had an increased rate of Parkinson's (and ischemic heart disease). 
Scientists took this a step further and examined a specific pesticide called chlorpyriphos on inner city children.  Their results showed clearly that chlorphyriphos exposure was correlated to ADHD. In other words, just one pesticide was able to very likely damage the brain enough to decrease dopamine and lead to classic ADHD symptoms. What are we doing to ourselves?
An even more toxic example is dieldrin, a DDT alternative, which has a study showing it increases oxidative stress, lowers dopamine levels and is likely a cause of Parkinson's Disease.  Furthermore, it has induced liver and hepatocellular cancers.  Even worse, it is not biodegradable. It has been banned for decades yet still persists in our food supply like radioactive waste.
NEWS FLASH: Chemicals in the environment are also be sabotaging our neurons as well. Researchers recently found that TCE (trichloroethylene), a very common chemical abundant in our environment (and in much of our drinking water supplies), is strongly linked to Parkinson's.  It is almost impossible to get away from TCE as it is used in dry-cleaning, carpet cleaners, paints, glues and degreasing agents.
SOLUTIONS? Researchers are hoping a new "maintenance" drug called isadiprine can protect us from Parkinson's and it should be in phase II trails as I write this. Parkinson's is actually likely partly genetic - they have found gene(s) that are strongly correlated - and environmental, i.e. pesticides, herbicides and other chemicals that we do not know about yet. However, I would argue that eating organic as much as possible would be wise until we better understand just what we are doing to our bodies.
There is also a Russian antihistamine (latrepirdine) of all things, that is holding great promise in the treatment of both Alzheimers and Parkinsons. 
1) Nora D. Volkow, M.D., director, U.S. National Institute on Drug Abuse, Bethesda, Md.; Andrew Adesman, M.D., chief, developmental and behavioral pediatrics, Schneider Children's Hospital, New Hyde Park, N.Y.; Sept. 9, 2009, Journal of the American Medical Association
2) Amer J of Epidemiology, 2007, 165(4):364-374,"Pesticide Exposure and Self-reported Parkinson's Disease in the Agricultural Health Study"
3) Neurology, 2008, 70:1461-1469, "Synuclein, pesticides, and Parkinson disease: A case control study"
4) National Acad of Sciences, July 24 2009, "LIMITED DATA SUGGEST POSSIBLE ASSOCIATION BETWEEN AGENT ORANGE EXPOSURE AND ISCHEMIC HEART DISEASE AND PARKINSON'S DISEASE IN VIETNAM VETERANS", http://www8.nationalacademies.org/onpinews/newsitem.aspx?RecordID=12662
5) Pediatrics, Dec 2006, 118(6):e1845-e1859, "Impact of Prenatal Chlorpyrifos Exposure on Neurodevelopment in the First 3 Years of Life Among Inner-City Children"
6) Exp Neurol, Apr 2007, 204(2): 619 630, "Dieldrin exposure induces oxidative damage in the mouse nigrostriatal dopamine system"
7) Hayes WJ jr. and Laws ET jr. eds. (1991) Handbook of Pesticide Toxicology, Academic Press Inc., San Diego, 732-735, 741, 828, 832, 836-840
8) Annals of Neurology, Article first published online: 14 NOV 2011, "Solvent exposures and parkinson disease risk in twins"