Explanation: Concentrated marijuana can be up to four times stronger in THC content than high grade or top shelf marijuana. Normal marijuana strains have a level of THC around 20 percent.  The THC level in concentrates can range between 40 and 80 percent.

Source: DEA, Drugs of Abuse 

Source: DEA, Drugs of Abuse
 

Explanation: The pesticides/poisonous solvents are left over from the extraction process.

Explanation:  One popular extraction method uses butane, a highly flammable solvent. This method is dangerous because butane is a very explosive substance. There have been explosions in houses, apartment buildings and other locations where someone tried the extraction. 

Source: DEA 

Being a highly concentrated form of marijuana, the effects upon the user may be more psychologically and physically intense than plant marijuana use. To date, long-term effects of marijuana concentrate use are not yet fully known. (Source: Vaping & Marijuana Concentrates)