Atmospheric particulate matter adhesion onto pollen: a review

From anther dehiscence to its atmospheric journey, airborne pollen grains can come into contact with other particulate matter (PM) and the external wall of pollen grains can be polluted by adhered atmospheric particles. Pollen and PM are some of the most common asthma triggers. The aim of this review was to discuss literature reporting pollen–particle complexes in the outdoor environment so their potential role in enhancing the risk of allergy could be examined. Twenty-five studies report pollen–particle complexes observed in outdoor conditions. PM adhesion onto the pollen surface was investigated in the literature by three different experimental approaches: direct collection from anthers, bulk exposure of pollen to PM in polluted areas, and direct sampling of airborne pollen during pollination. Four processes are proposed to explain deposition of PM on the surface of pollen grains: sticking of particles to anthers and pollen during dehiscence, airborne pollen-particles coagulation, wet and dry scavenging of pollen and particles, and co-deposition of PM and pollen in the respiratory system. Despite potential adverse health effects, the magnitude of pollution of pollen by PM is poorly understood. To address this issue, we suggest the following guidelines for research: (1) Possible artifacts during sampling and handling of anthers must be carefully avoided; (2) Control pollen grains (i.e. non-polluted) should be carefully selected; (3) A better knowledge of pollen atmospheric deposition and resuspension is needed; (4) The health effects of pollen-particles complexes need more attention regarding symptoms and allergic sensitization; (5) Use of individual pollen samplers could give a depiction of the degree of pollution of pollen that is actually inhaled.