This is confusing, but the rule states that “a” should be used before words that begin with consonants (e.g. b, c ,d) while “an” should be used before words that begin with vowels (e.g. a,e,i). But, the tricky part is that which one you use is determined by the pronunciation and not by the spelling, as many people wrongly assume.
You would therefore say “an hour” (because hour begins with a vowel sound) and “a history” (because history begins with a consonant sound).
Similarly you should say “a union” even if union begins with a “u”. That is, because the pronunciation begins with “yu”, which is a consonant sound.
So, the question arises, would that mean it would be “an” ’erbalist in the USA because they don’t pronounce the “h” and “a” herbalist in countries which do pronounce the “h”?