There are several causes of chronic cough and the presence of other symptoms can help to reach a final diagnosis. If this cough worsens or your mother begins to to become short of breath, febrile or begins to cough up blood at any time, then she needs to be seen by her doctor.
That being said, the main causes of a chronic, nagging cause without any other accompanying bothersome symptoms that I discuss with my patients are: post-nasal drip, GERD, medication effects, and
asthma. In response to your question, post nasal drip is often be related to allergy and may indeed be the only symptom of an environmental allergy.
Chronic post nasal drip is normally due to inflammation in the nasal passages from either allergies (allergic rhinitis) or from a viral
illness (infectious or post infection rhinitis). It is common to have post nasal drip as the only sign of allergy and likewise, post nasal drip can linger long after a viral upper respiratory tract infection. What causes the cough? It is the irritation caused by the drainage of mucous down the posterior pharynx and it is also common for the rhinitis to be so subtle that the only symptom a patient has is cough (i.e. no runny nose, sneezing etc). the treatment for allergic rhinitis is steroid nasal spray (e.g. flonase) or a non-sedating anti-histamine (e.g. claritin or zyrtec). if the issues is infectious/post infectious rhinitis, the symptoms will gradually improve and in the meantime a nasal spray like flonase can help.