Often a library will assign a broader heading to these. When the appropriate narrower heading is established, the library will generally not go back and search for earlier works for which the heading would have been appropriate. One way around this problem is for catalogers who encounter a new work, which needs, a new subject heading is to send a subject-heading request to the Library of Congress through the SACO program. LC has online work forms, which may be downloaded and filled out to expedite the establishment of subject headings. This is an effective way to assign the most appropriate heading to the work, rather than assigning broader or related topics.
Headings often change as well.
If authority control work is neglected in a library's catalog, split files may occur. Many works with the old heading will still be in the catalog with works under the new heading. There is usually a cross-reference in the record from the old heading to the new, but not always. It is impossible to compile a listing of all environmental subject headings. As noted above, new headings are being added all of the time so this list is already out of date before it is published.