Causes

• Some studies suggest that being part of a large family or attending day care increases the risk for early respiratory infections but reduces the risk of childhood asthma. The occasional cold, then, may be protective.
• In a 2002 study, researchers measured levels of bacterial byproducts called endotoxins in the mattress dust of 812 children. Those with the highest levels had an 80% lower rate in allergies and asthma.
• Another study further found a strong association between allergy development and the absence of certain beneficial bacteria (called probiotics) carried infants' intestines. Infants who were born in more hygienic environments tended to lack these bacteria. Antibiotic overuse and modern hygiene may reduce these helpful organisms. (Probiotics are available in active yogurt cultures and in supplements, which are being studied for protection.)

According to many studies, the standard vaccinations against serious childhood infections pose no risk for developing asthma or hay fever. Some studies have even reported lower risk for asthma and allergies in the second and third years after vaccinations..

Overexposure to Indoor Allergens

Some evidence suggests that the increase in allergies and asthma may be due to overexposure to indoor allergens. These may include wall-to-wall carpeting, cats, and mold produced by dampness in homes. Children who spend hours indoor each day may become overexposed to indoor allergens.  This exposure is intensified by the recent trend of making homes more energy-efficient, which may result in more dust mites being trapped inside. However, other studies suggest that early exposure to allergens may actually prevent the development of allergies in children.

Triggers of Seasonal Allergic Rhinitis (Hay Fever or Rose Fever)

Seasonal allergic rhinitis occurs only during periods of intense airborne pollen or spores. It is commonly, although inaccurately, called hay fever or rose fever, depending on whether it occurs in the late summer or spring. No fever accompanies this condition, and the allergic response is not dependent on either hay or roses. In general, triggers of seasonal allergy in the U.S. include:

• Ragweed. Ragweed is the most dominant cause of allergic rhinitis in the US, affecting about 75% of allergy sufferers. One plant can release one million pollen grains a day. Ragweed occurs everywhere in the U.S., although it is less common in western coastal states, southern Florida, northern Maine, Alaska, and Hawaii. The effects of ragweed in the northern states are first felt in middle to late August and last until the first frost. Ragweed allergies tend to be most severe before midday.
• Grasses. Grasses affect people in mid-May to late June. Grass allergies are experienced more in the late afternoon.
• Tree Pollen. Small pollen grains from certain trees usually produce symptoms in late March and early April.
• Mold Spores. Mold spores that grow on dead leaves and release spores into the air are common allergens throughout the spring, summer and fall. Mold spores may peak on dry windy afternoons or on damp or rainy days in the early morning.