Some samples of the best "bho" i have seen were <25 ppm to the worst samples exceeding 1500 ppm and thats honesty a big difference.
Mainly this happens because some people try to make "bho" at home and haven't properly educated themselves and dont understand the concept purging solvents from extracts properly. Also the use of n-butane works wonders.
To achieve higher thc percent and a clean extract with low residual solvent, "bho" also needs to be winterized.
After blasting the herb with butane, the liquid butane and oil mixture, has pure grain alcohol added to quickly react and remove most of the butane. This remaining alcohol mixture is then cold filtered to remove the plant waxes/contaminants out of the concentrate/extract.
Once winterized the extract still needs to be properly purged of the remaining solvents Used in extraction. Usually this is done via vacuum chamber or vacuum oven.
So after about 24 to 48 hrs of cold filtering, then about 24 to 72 hours of vacuum purging, the conconcentrate is very stable and clean to smoke or even eat if decarbed.
Ps
Many samples tested @ analytical 360 were below 25ppm of residual solvent. That works out to 99.9975% free of solvents. Or possibly better as the cut off is 25ppm. Just some light on the subject.
The daily allowable limit for butane in herbal extracts in the USA is 5000 ppm per day.
Put it this way. Really dirty bho at 500ppm would be safe even if the person took ten .1g dabs per day.
http://www.ahpa.org/Default.aspx?tabid=223#section_solvents