This year we have noticed a paper documenting a much larger study from 1970 on this subject - quoting the summary below:
A history of respiratory or other allergic symptoms during the Christmas season is occasionally obtained from allergic patients and can be related to exposure to conifers at home or in school.
Incidence and mechanism of production of these symptoms were studied. Of 1657 allergic patients, respiratory and skin allergies to conifers occurred in 7%. This seasonal syndrome includes sneezing, wheezing and transitory skin rashes. The majority of patients develop their disease within 24 hours, but 15% experience symptoms after several days delay.
Mould and pollen studies were carried out in 10 test sites before, during and after tree placement in the home. Scrapings from pine and spruce bark yielded Urge numbers of Penicillium, Epicoccum and Alternaria, but these failed to become airborne. No significant alteration was discovered in the airborne fungi in houses when trees were present. Pollen studies showed release into air of weed, grass and tree pollens while Christmas trees were in the house. Oleoresins of the tree balsam are thought to be the most likely cause of the symptoms designated as Christmas tree allergy.