A study published in Allergy reports a higher incidence of hay fever allergies in adults among those that swam while they were younger. The study group used interviews and questionnaires to gather the medical and swimming pool attendance history of over 2,500 adults. The analyzed data revealed a significant correlation between swimming in swimming pools and reported occurrences of hay fever allergy problems. The strongest correlation found was between swimming in childhood and occurrences of hay fever in adult life.
The study concluded that exposure to chemicals in swimming pools was the likely culprit. The subject's lung integrity could have been compromised by chlorine by-products in swimming pool water. This meant that allergens had a greater chance to affect the lungs surface, and this could result in greater incidences of hay fever allergy problems. The correlation was probably higher between childhood swimming and adult hay fever allergies because the adult that swam as a child probably had more exposure to swimming pool water chemicals (like chlorine and chlorine by-products) over their lifetime.
Kohlhammer, Y., Doring, A. Schafer, T. Wichmann, H.-E. Heinrich, J. The KORA Study Group. "Swimming pool attendance and hay fever rates later in life." Allergy.. 61(11):1305-1309, November 2006.