This is a great question. Adults who have never had an allergy in their life can develop new allergies. They can also experience worsening allergy symptoms as they age. The reasons for this are not completely clear. The contributing factors include:
- Genetic predisposition for allergies
- Family history of allergies (e.g., eczema, hay fever, food allergies, asthma)
- Changes in the environment including spending more time indoors at home
- Changes to the diet
Approximately 10% of adults have a food allergy. Fifty percent of them developed it in adulthood. For example, some adults can eat shellfish all their life without problems. They can then suddenly experience allergy symptoms after eating shrimp. These individuals may have had a genetic predisposition to this allergy. It took years of exposure to trigger this response.
New-onset environmental allergies (e.g., tree, grass, and/or weed pollen) can be related to moving to a new region or environment. Also, climate change has contributed to longer and more intense pollen seasons. This can have an impact here.
Adults can also develop new allergies to common indoor allergens (e.g., pets, dust mites, mold spores). For example, spending more time indoors at home could lead to environmental allergies. This is in adults susceptible to developing allergies. This could happen with exposure to pets, dust mites, and/or mold spores. Finally, getting a new pet in the home can also have an impact.