Cannabis (hemp) positive skin tests and respiratory symptoms

We have noted several patients who had rhinitis and/or asthma symptoms when exposed to Cannabis plants in the summer months. Cannabis plants are common in the Midwest.

To examine whether Cannabis might be a clinically important allergen, we determined Cannabis pollination patterns in the Omaha area for 5 years, the prevalence of skin test positivity, and the association with respiratory symptoms.

Airborne Cannabis (and other weed) pollens were collected using a Rotorod air impactor, and pollen counts were done using a standardized protocol.

Measurable Cannabis pollen count was not recorded until the last 2 weeks of July. Peak pollination typically occurred during mid- to late-August, and comprised up to 36% of the total pollen counts. Cannabis pollen was not observed after mid-September. To determine the prevalence of skin test positivity, we added Cannabis to the multi-test routine skin test battery. Seventy-eight of 127 patients tested (61%) were skin test positive. Thirty of the 78 patients were randomly selected to determine if they had allergic rhinitis and/or asthma symptoms during the Cannabis pollination period. By history, 22 (73%) claimed respiratory symptoms in the July through September period. All 22 of these subjects were also skin test positive to weeds pollinating during the same period as Cannabis (ragweed, pigweed, cocklebur, Russian thistle, marsh elder, or kochia).

The strong association between skin test reactivity, respiratory symptoms, and pollination period suggests that Cannabis could be a clinically important aeroallergen for certain patients and should be further studied.