LC headings have been widely criticized in the past for using outdated and arcane terminology and being slow to establish new headings even when a topic has considerable literature. That has changed in recent years, and LC has recently been more willing to create subject headings while a topic is still new and change outdated terminology to more current forms. Still, there are some quirks in LC's manner of establishing and assigning subject headings. Often headings seem to be at variance with common usage. However, it is easy to forget that common usage is quite imprecise and ambiguous while LC aims for consistency in its headings.
Usage also changes over the years.
The modern connotation of the word Environment in popular usage has a much narrower focus than in its former usage, where it referred not only to the natural environment, but also to any physical environment. Because of this ambiguity, LC has never established Environment as a heading, preferring to use it as a reference to other more specific headings. The word Ecology in common usage has a meaning similar to that of Environment, whereas in the scientific community it has a much more specific meaning, which it retains in LC practice. It is sometimes as difficult to predict what heading will be assigned to a work as it is to predict what search words are used. Persistence is required of the user and the reference librarian.