Permeability is measured as the rate at which a particular substance in solution crosses the membrane.
Over the years, any number of coincidental findings have suggested that exposure to a particular substance may cause a certain illness.
THE problems associated with teenage substance abuse extend beyond the specific health risks associated with a particular substance, like tobacco or alcohol.
"It's almost unheard of for N.I.H. to have as a goal the production of a particular substance like a vaccine," Dr. Baltimore said.
It's not that this was a debate of any particular substance, as opposed to the merely "symbolic" battles we suffer through now.
In most cases, you can see that the 'higher' a respondent's socio-economic status, the more likely they are to associate a particular substance with a "drug problem".
An experiment on taste perception might invite ratings of the taste intensity of a particular substance from 1 ("very weak") to 9 ("very strong").
And, regulators may come under pressure to tighten pollution standards to protect the small fraction of the population that is most sensitive to a particular substance.
Wada recognises this need – that meldonium is a particular substance, which has created an unprecedented situation and therefore warranted additional guidance for the anti-doping community".
"Given the amount of suspensions we've seen this year, we have made a greater emphasis on making players aware of the policy on this particular substance".
This is not to say that chemicals are evil, and in many cases the evidence against a particular substance is balanced by other studies that are exonerating.
