Cases of skin irritation and sensitization, local rashes or chemical burns can over time lead to skin allergy. Repeated or long term-inhalation of certain substances, causing respiratory irritation, could potentially lead to allergic asthma.
Anyone who is exposed to a high dose of a substance is inclined to form an allergy to that particular substance. Case studies have shown that workers subjected to repeated and prolonged exposure to certain substances also have a higher amount of allergy to those substances.
Allergies may also develop from repeated low-level exposure from chemicals in products. The products, or different products containing the same substances, may have been used for a long time, even for many years, before any reaction occurs. The usual case in this regard is with products that are applied to the skin which, over time, leads to skin sensitization issues. The release of the allergy may be delayed but once an allergy is developed it cannot be fully cured.
There are ways to reduce the sensibility, via for example immunotherapy, but the cure is basically to stay away from ingredients that trigger allergic reactions. This could be tricky as chemicals often have multiple applications. Some are used both as fragrance agents in cosmetics and as flavoring agents in food. Furthermore, exposure to the same chemicals in other settings, in for example surgery, may also trigger allergic reactions, thus causing wider issues.
If you get a reaction from using certain products or ingredients, go see a health care specialist to get a patch test. It is good for you to have the facts, and it is also good to add to the allergy statistics. Ask to also be tested for oxidized perfumes (read below if you want to know what they are!)