Table 2.

Characteristics of studies ordered by study design (timeseries and case-crossover studies, panel studies, cohort study) and year.

Author, year	Type of study	Population, location	Time period	Outcome	Allergens	Air pollutants	Allergn - pollutant interactions	Atopic status
Guilbert et al., 2018	Timeseries	Residents of Brussels-Capital Region, Belgium	01/01/2008 to 31/12/2013	Daily number of asthma hospital admissions	Daily counts of 11 pollen taxa (alder, hazel, yew, Cupressaceae, ash, hornbeam, birch, oak, plantain, grasses, mugwort) and two fungal spore taxa (Alternaria, Cladosporium)	NO2, O3, PM2.5, PM10	Interactions tested between all allergens and air pollutants. Significant interactions found, notably risks higher for grass and birch pollen counts on days with high PM10 and O3 levels, respectively (p = 0.05). Interactions also found in the unexpected direction between Alternaria and PM10 and PM2.5, oak and NO2 and hazel and O3 (p = 0.06).	No information
Phosri et al., 2017	Case-crossover	Residents of Fukuoka City, Japan, attending 10 otolaryngology and ophthalmology clinics	2002 to 2012 (01/02–30/04 only)	Daily number of medical consultations for pollinosis	Daily counts of pollen (comprises Japanese cedar and cypress pollen)	PM2.5 mass and it's components (NO3−, non-sea-salt-SO42−, sea-salt -Ca2+)	Interactions tested between pollen and all pollutants. Cumulative effect of pollen greater on days with high non-sea-salt Ca2+ (p < 0.01) and, for some models, non-sea-salt SO42 and NO3−. Pollen effects (unexpectedly) greater on days with moderate (compared to high) total PM2.5 mass (p < 0.01).	No information
Sakata et al., 2017	Timeseries	Residents of Fukuoka City, Japan, attending four otorhinolaryngology clinics	1989 to 2012 for Clinic I and III, 1994–2012 for Clinic II, 1989–2001 for Clinic IV (February to April only)	Daily number of medical consultations for pollinosis	Daily counts of pollen (comprises Japanese cedar and cypress pollen)	Asian dust days (visibility <10 km), SPM	Interactions tested between Asian dust days and pollen and were consistently significant for all lags in 3 out of 4 clinics (p < 0.01). Interactions also tested between SPM and pollen and were significant for all four clinics during Asian dust-free days (p < 0.01).	No information
Tham et al., 2017a	Case-crossover	Asthmatic children (2–18 years) attending Campbelltown, Camden or Liverpool hospitals, Sydney, Australia	29/05/2008 to 03/05/2013	Daily number of asthma hospital admissions	Counts of total fungal spores and 20 fungal spore taxa	NO2, O3, PM2.5, PM10	Interactions tested between pollutants and allergens. None significant.	No information
Tham et al., 2017b	Case-crossover	Melbourne Air Pollen Children and Adolescent (2–17 years) study participants (hospitalized for asthma in Melbourne, Australia)	09/2009 to 12/2011	Daily number of asthma hospital admissions	Counts of total fungal spores and 12 fungal spore taxa (Alternaria, Cladosporium, Ganoderma, Leptosphaeria, Pleospora, Sporormiella, Pithomyces, smuts, Coprinus, Drechslera, Stemphylium, Periconia)	NO2, O3, PM2.5, PM10	Interactions tested between pollutants and allergens. None significant.	Information available for 630 participants. Analyses stratified by sensitization (8.9% to Alternaria only, 6.5% to Cladosporium only, and 13.7% to both).
Chen et al., 2016	Timeseries and case-crossover	Residents of Adelaide, South Australia	01/07/2003 to 30/06/2013	Daily number of asthma hospital admissions	Daily total pollen counts	NO2, O3, PM2.5, PM10,	Unclear which interactions tested but reported that none significant.	No information
Gleason et al., 2014	Case-crossover	Children (3–17 years) in New Jersey, USA	2004 to 2007 warm season (01/04 to 30/09 only)	Daily number of asthma emergency department visits	Daily counts of tree pollen, weed pollen, grass pollen and ragweed	O3, PM2.5	Stated that interactions tested between quintiles of O3 and PM2.5 with high vs low categories of each pollen type, although no interaction terms formally reported. Stratified models indicate effect of O3 and PM2.5 greater on days with high 3-day average weed pollen counts.	No information
Konishi et al., 2014	Case-crossover	Residents of Chiyoda ward, Tokyo, Japan, attending one general practice	2001 to 2011 (January to May only)	Daily number of initial doctor consultations for pollinosis	Daily counts of pollen (comprises Japanese cedar and cypress pollen)	CO, NO2, O3, PM2.5, SO2, SPM	Interactions tested between pollen counts and PM2.5 and SPM. Effect of 5-day average cumulative pollen count greater on days with high PM2.5 (consistently) or SPM levels (less consistently), compared to days with moderate pollutant levels. Results less clear when considering single-day pollen effects.	No information
Cakmak et al., 2012	Timeseries	Residents of 11 cities in Canada.	01/04/1994 to 31/03/2007	Daily number of asthma hospital admissions	Daily counts of weed, trees and grass pollen and Basidiomycetes, Ascomycetes and Deuteromycetes fungal spores.	CO, NO2, O3, PM2.5, PM10, SO2	Interactions tested between all pollutants and all allergens. Effect of allergens greater on days with high air pollution levels in several cases, especially for PM10. Interaction terms significant (p < 0.05) between ascomycetes and CO and PM10, basidiomycetes and CO, NO2, SO2 and PM10, deuteromycetes and CO, NO2, SO2 and PM10, trees and PM2.5, weeds and PM10	No information
Darrow et al., 2012	Case-crossover	Residents of Atlanta metropolitan area, USA, attending 41 acute-care hospitals	1993 to 2004 Analyses limited to pollen season for each taxa (2–5 months).	Daily number of emergency department visits for asthma and wheeze	Daily counts of four tree pollen taxa (Betulaceae, Cupressacea, Quercus, Pinaceae), grasses and Ambrosia	CO, NO2, O3, PM2.5, PM10, SO2	Interactions between O3 and Quercus and grass pollen count tested, but neither significant.	No information
Erbas et al., 2012	Timeseries	Children (<15 years) living in Melbourne, Australia	01/09/2003 to 31/12/2003	Daily number of asthma emergency department visits for asthma	Daily counts of grass pollen	NO2, O3, airborne particle index	Interactions tested between grass pollen count and the Airborne Particle index and NO2. Neither significant.	No information
Ghosh et al., 2012	Timeseries	Residents of Kolkata, India, with a diagnosed respiratory allergy attending a hospital allergy clinic (catchment area of 2 hospitals included)	2004 to 2009	Number of asthma-related hospital admissions recorded in 10-day time-slots.	Daily counts of five pollen types (Areca, Carica, Cheno-Amnthaceae, Cocos, Phoenix, Cyperaceae, Poaceae), 7 fungal types (Alternaria, Aspergilli, Basidiospores, Cladosporium) and total fungal spores	NO2, SO2, SPM, respirable particulate matter, O3 (latter only for two years)	Unclear which interactions tested but reported that none significant.	No information
Krmpotic et al., 2011	Timeseries	Residents of Zagreb, Croatia	01/01/2004 to 31/12/2006	Daily number of asthma hospital admissions	Daily alder, hazel, birch, hornbeam, oak, grasses and ragweed pollen counts	CO, NO2, PM10	Interactions tested between all pollutants and pollen variables. None significant.	No information
Babin et al., 2008	Timeseries	Residents of Washington, DC, USA, on Medicaid health plan	01/10/1994 to 22/11/2005	Daily number of asthma-related acute care visits	Daily tree, grass, weed pollen counts and total mold spore counts.	O3, PM2.5, PM10	Unclear which interactions tested but reported that none significant.	No information
Babin et al., 2007	Timeseries	Residents of Washington, DC, USA.	10/2001 to 09/2004 (except 12/2002)	Daily number of asthma-related pediatric emergency department visits and admissions	Daily tree, grass, weed pollen counts and total mold spore counts	O3, PM2.5	Interactions tested between both pollutants and all allergens but none significant.	No information
Carracedo-Martinez et al., 2008	Case-crossover	Residents of the City of Vigo, Spain	1996–1999	Daily number of medical emergency calls for cardiovascular and respiratory causes	20 types of pollen counts	Black smoke, SO2	Unclear which interactions tested but reported that none significant.	No information
Villeneuve et al., 2006	Timeseries	Elderly (≥65 years) residents of Toronto, Canada.	01/01/1995 to 31/12/2000	Daily number of primary care visits for allergic rhinitis	Daily ragweed counts	NO2, O3, PM2.5, PM10, PM2.5–10, SO2	Interactions tested between ragweed and all pollutants except O3. None significant.	No information
Dales et al., 2004	Timeseries	Residents of 10 cities in Canada.	01/04/1993 to 31/03/2000 (only months with pollen/fungal counts >0)	Daily number of asthma hospital admissions	Daily counts of weed, trees and grass pollen and Basidiomycetes, Ascomycetes and Deuteromycetes fungal spores.	NO2, O3, SO2, coefficient of haze	In combined analyses, only interaction term between O3 and tree pollen count was significant (p < 0.05). City-specific interactions also observed between O3 and certain allergens (basidiomycetes in 3 cities, deuteromycetes in 3 cities, weeds in 3 cities, trees in 4 cities, grass in 1 city).	No information
Lierl and Hornung, 2003	Timeseries	Residents of Cincinnati, USA	04/1996 to 10/1996 and 04/1997 to 10/1997 (08/1996 excluded)	Daily number of asthma hospital visits (includes visits and admissions combined)	Daily counts of total pollen and fungal spores	O3, PM10	Interactions tested between pollen counts and air pollutants. No interaction terms formally reported. Stratified models indicate effect of pollen counts greater on high (> 33 μg/m3) PM10 days.	No information
Sunyer et al., 2002	Case-crossover	Residents of Barcelona, Spain, who attended emergency department for asthma between 1985 and 1989 in the four largest hospitals (covers 80% of population)	1985 to 1995	Number of overall and cause-specific mortality (including respriatory mortality)	Weekly counts of total pollen, grass pollen, total fungal spores, Cladosporium, Alternaria, Epicoccum and Helminthosporium	Black smoke, CO, NO2, O3, SO2	Interactions tested between pollutants and allergens. None significant.	No information
Weisel et al., 2002	Timeseries	Residents of central and northern New Jersey, USA	01/05/1995 to 31/08/1995	Daily number of asthma hospital visits and admissions	Daily counts of total pollen and total fungal spores	O3	Unclear which interactions tested but reported that none significant	No information
Lewis et al., 2000	Timeseries	Residents of Derby, England	01/1993 to 12/1996	Daily number of asthma hospital admissions and visits (assessed separately)	Daily counts of grass and birch pollen, hyaline basidiospores, coloured basidsiospores, Didymella, Alternaria, and Cladosporium	Black smoke, O3, NO2	Interactions tested between all pollutants and pollen variables. None significant.	No information
Anderson et al., 1998	Timeseries	Residents of London, UK	1987 to 1992	Daily number of asthma hospital admissions	Daily birch, grass and oak pollen counts	Black smoke, NO2, O3, SO2,	Interactions tested between all three allergens and pollutants significant in unipollutant models (O3 and SO2), restricted to warm season. Significant interaction found between SO2 and grass pollen counts in 0–14 age group in warm season (p < 0.01). Interactions also found between ozone and birch and ozone and oak in unexpected direction in all-ages analysis in the warm season (p < 0.001)	No information
DellaValle et al., 2012	Panel	Children (4–12 years) with asthma living in Connecticut, Massachusetts and New York, USA	Recruited 2000 to 2003, each followed for one year.	Daily record of asthma symptoms of wheeze, night symptoms, shortness of breath, chest tightness, persistent cough and rescue medication usea	Daily counts of total pollen, and tree, grass and weed pollen	NO2, O3, PM2.5, SO2	Interactions tested between O3 and all pollen variables, but none significant.	Information available for 319 (74%) subjects. Analyses stratified by sensitization to grass (26%) and weed (22%) pollen.
Chen et al., 2011	Panel	Children (mean age 10.6 years) of three elementary and two middle schools in Taipei Country, Taiwan	09/2007 to 06/2008	Monthly FVC, FEV1, FEF25, FEF50, FEF75, FEF2575	Daily total fungal spore counts in week of lung function measurement	CO, NO2, O3, PM2.5, PMcoarse, SO2	Unclear which interactions tested but reported that none significant.	No information
Jalaludin et al., 2004	Panel	Primary school children (mean age 9.6 years) with a history of wheeze attending six primary schools in western and southwestern Sydney.	01/02/1994 to 31/12/1994	Daily record of respiratory symptoms (wheeze, wet cough, dry cough), asthma medication use, visit to a doctor for asthmaa	Daily counts of total pollen and Alternaria	NO2, O3, PM10	Interaction terms tested between air pollutants and counts of total pollen and Alternaria, but none significant.	No information
Delfino et al., 2002	Panel	Children/adolescents (9–19 years) with asthma living in Alpine California, USA	01/03/1996 to 30/04/1996	Daily record of asthma symptoms that interfere with daily activitiesa	Daily counts of total pollen and fungal spores	NO2, O3, PM10	Interactions tested between pollutants and allergens but none significant.	77.3% sensitized to pollens or fungi.
Just et al., 2002	Panel	Children (mean age 10.9 years) diagnosed with asthma who are outpatients at a hospital in Paris, France	01/04/1996 to 30/06/1996	Daily record of incident and prevalent asthma attacks, nocturnal cough, wheeze, irritation symptoms, respiratory infections, use of β2-agonist and morning and daily variability in PEFRa	Daily counts of total pollen	Black smoke, NO2, O3, PM13 (SO2 available but levels too low for analysis)	Interaction terms tested between O3 and pollen counts. Interaction term significant for incident asthma attacks (p = 0.002).	90.2% were atopic.
Ross et al., 2002	Panel	Asthmatics living in East Moline, USA	24/05/1994 to 25/10/1994	Daily record of morning and evening PEFR, symptom scores frequency of asthma attacks and asthma medication usea	Daily counts of total, grass and ragweed pollen, as well as counts of total, Alternaria, Cladosporium, Curvularia, Drechslera and Epicoccum spores.	O3, PM10 (only every 6th day), SO2	Interactions tested between O3 and allergens. None significant.	41–46% sensitized to trees, grasses or ragweed. Analyses stratified by sensitization.
Higgins et al., 2000	Panel	Asthmatics or those with chronic obstructive pulmonary disease who react to methacholine attending two general practices in Halton Health District, UK	28 days from August to mid-September 1991	Daily mean and variability in PEFR, presence of wheeze symptoms (data for dyspnoea, cough, throat irritation, eye irritation not usable for analyses)a	Daily counts of total fungal spores (measured pollen counts too low for analyses)	NO2, O3	Interactions tested between total fungal spore counts and O3 and NO2. Interactions observed between fungal spores and O3 for daily mean and variability in PEFR and wheeze symptoms (p < 0.02). Interaction also found between fungal spores and NO2 for wheeze but in unexpected direction (p < 0.05).	63% atopic. Analyses stratified by sensitization.
Jalaludin et al., 2000	Panel	Primary school children (mean age 9.6 years) with a history of wheeze attending six primary schools in western and southwestern Sydney	01/02/1994 to 31/12/1994	Daily record of evening PEFR (daily mean deviation of 3 measurements and maximum)	Daily counts of total pollen and Alternaria	NO2, O3, PM10	Unclear which interactions tested but reported that none significant	No information
Delfino et al., 1997	Panel	Asthmatics living in Alpine California, USA	09/05/1994 to 03/07/1994	Daily record of responses for asthma symptom severity and inhaler use and morning and evening PEFRa	Daily counts of total pollen and fungal spores, as well as 12 fungi types (Alternaria, Cladosporium, Helminthosporium, Aspergillus and Penicillium, Coprinus, Periconia, Botrytis, total ascospores, total basidiospores, hyphal fragments, rusts, unidentified spores).	O3, PM10	Interactions tested between pollen counts, PM10 and O3 with fungal spore counts for all outcomes, but none significant.	All subjects sensitized to ≥1 pollen extract. Analyses conducted among 16 subjects sensitized to ≥1 of five skin prick tests for fungal spore taxa.
Delfino et al., 1996	Panel	Children/adolescents (9–16 years) with asthma living in Mesa region in San Diego, USA	20/09/1993 to 31/10/1993	Daily record of responses for asthma symptom severity and inhaler usea	Daily counts of total pollen and fungal spores, as well as 5 fungi types (Alternaria, Cladosporium, Helminthosporium interseminatum, Aspergillus, Penicillium).	HNO1, HNO3, O3, PM2.5, sulfate and nitrate fractions of PM2.5	Interactions tested between O3 and the fungal count variables, but none significant.	All subjects sensitized to ≥1 pollen extract. Analyses conducted among 10 subjects sensitized to ≥1 of five skin prick tests for fungal spore taxa.
Jones et al., 1994	Panel	Children (mean age 10.6 years) with a respiratory disorder (95% had asthma) attending two private practice offices and one pediatric clinic in Baton Rouge, Louisiana	14/06/1990 to 30/09/1990	Daily record of PEFR, respiratory symptom count and rating, activity time and rating, asthma/COPD medication counta	Daily counts of total pollen and fungal spores	NO2, O3	Two-way interactions tested between both allergens and pollutants, as well as one-three way interaction between O3, NO2 and fungal spore counts. None significant.	No information
Kanatani et al., 2016	Longitudinal cohort	Pregnant women (taking part in a larger cohort study) living in Kyoto, Toyama and Tottori, Japan	10/2011 to 11/2011, 02/2012 to 05/2012, 10/2012 to 11/2012, 02/2013 to 05/2013	Any allergic symptom (nasal, ocular or bronchial; symptom score > 0)a	Daily counts of pollen (comprises Japanese cedar and cypress pollen)	Asian dust days (dust level ≥ 0.07/km) compared to randomly selected control days (dust level < 0.07/km)	Interaction term tested between dust days (yes/no) and pollen counts, and was significant (p < 0.001)	57.2% sensitized to Japanese cedar pollen. Analyses stratified by sensitization.

CO = carbon monoxide; NO₂ = nitrogen dioxide; O₃ = ozone; PEFR: peak expiratory flow rate; PM_2.5 = particulate matter ≤2.5 μm in aerodynamic diameter; PM₁₀ = particulate matter ≤10 μm in aerodynamic diameter; SO₂ = sulphur dioxide; SPM = suspended particulate matter.

^aSelf-reported.