I don't know the meaning of the phrase in this sentence
We are seldom exposed only to a single contaminant in the environment-but more often than not to a cocktail of chemical mixture.
How to understand 'more often than not'?
I would think of this phrase by rephrasing and making it a sentence like, "There are more often times when we are exposed to a cocktail of chemical mixture than the times when we are not exposed to one. More often than not=as often as not=usually. Hope this helps...
"More often than not" describes a degree of probability which is greater than 50%.