The distribution of airborne pollen grains was assessed daily at eight locations nationwide between July 1, 1997 and June 30, 2009. A Burkard 7-day sampler (Burkard Manufacturing Co Ltd, Hertfordshire, UK) was installed at a height of 1.5 meters above sea level. The pollen collection areas were (1) Hanyang University Seoul Hospital and Seoul Meteorological Station in the Seoul metropolitan area, (2) Hanyang University Guri Hospital in the north-central region of South Korea, (3) Kangneung Asan Hospital and (4) Kangwon National University Chuncheon Hospital in the north-east, (5) Daejeon Meteorological Station in the central, (6) Kwangju Veterans Hospital and (7) Jeonju Jesus Hospital in the south-west, (8) Daegu Fatima Hospital in the south-east, (9) Busan St. Maria Hospital and (10) Ulsan Dongkang University in the far south-east, and (11) Jeju University Hospital on Jeju Island, the most distant of the southern islands (Fig. 1). Airborne pollens were collected every day from all samplers at all collection sites. The samples were sent every week to Hanyang University Guri Hospital, which was responsible for determining pollen concentrations. After each glycerin adhesive plastic was stained with Calberla's fuchsin staining solution (glycerin 10 mL, 95% alcohol 20 mL, distilled water 30 mL, and basic fuchsin 0.2 mL), the number of pollen grains per m³ was calculated at a magnification of ×400.19 Pollens were morphologically identified and classified by their size, color, pore shape, and surface pattern, with reference to the distribution of nationwide allergy-related plants such as trees, grasses, and weeds.20 Weather conditions on each collection day were recorded throughout the collection period and their associations with pollen distribution were assessed.

Pollens are spores derived from stamens and can be divided into trees, grasses, and weeds. In Korea, allergenic pollens appear at temperatures ≥10℃ between February and November. The peak seasons are spring and fall; thereafter concentrations of these pollens decrease abruptly. Tree pollens are mainly observed between March and May, grass pollens between May and September, and weed pollens between August and October. Trees include pine, oak, alder, and birch, of which pine accounts for 70% of all tree pollens. Grass pollens are difficult to discriminate from other pollens using light microscopy; detailed classification of these species can be accomplished using electron microscopy. Japanese hop, mugwort and ragweed belong to this family.

Pollen concentrations are closely related to general weather conditions, such as temperature and rainfall. Temperature and rainfall are particularly important determinants of pollen concentration. Pollens are heavily concentrated at 10-30℃, which is the most suitable temperature for plant growth (Fig. 3). High concentrations of pollen occur on days without rainfall. More pollen grains are airborne when the atmospheric temperature rises immediately after rainfall.

Since the 1900s, remarkable climate changes have occurred due to rapid industrialization. Atmospheric temperature has increased due to the greenhouse effect secondary to the increased atmospheric CO₂ concentration. Weather conditions, including CO₂ concentration, rainfall, atmospheric temperature, humidity, wind speed, and wind direction may alter the concentrations of plant pollens and other allergens, which can subsequently influence the occurrence of asthma or allergic rhinitis. Many studies have demonstrated that CO₂ concentration and increased atmospheric temperature increase pollen concentration. Ziska et al.21,22 reported that the atmosphere contains high levels of CO₂ concentration due to rapid industrialization. They also predicted that pollen levels would rise due to increased CO₂ concentration. Previous studies have demonstrated a significant relationship between pollen level and elevated atmospheric temperatures, and that pollen concentrations are higher in cities that have both a high atmospheric temperature and a high CO₂ concentration than rural areas.23,24

A characteristic of climate change is that flowers blossom earlier and fall later, subsequently extending the duration of pollen production. For this reason, the annual amount of pollen has increased. It is well known that the distribution of plants, as well as the allergenicity of their pollen, have continuously changed. When subjects are sensitized to airborne pollens and are exposed to increased air pollution, allergic disease activity increases and clinical symptoms are aggravated. Such changes represent a serious global public health concern. To minimize the occurrence of clinical manifestations in allergic patients, a pollen calendar for Korea was developed between 1997 and 2002 (Fig. 4). However, because this calendar was based on the mean concentration of pollen nationwide, it did not reflect the regional characteristics of pollens and may be inaccurate due to recent changes in the climate. Therefore, it was necessary to develop a new calendar that included the geographic and monthly distribution of pollens and included data obtained after 2004.25 The old calendars were also modified by deleting data that did not match the dates of pollen distribution as reported in previous research.

A new pollen calendar was developed based on the pollen concentration data obtained in the eight locations listed above between 1997 and 2009. Briefly, pine was the most common source of tree pollens. Overall, tree pollens were detected at higher concentrations than grass or weed pollens. Pine, alder, oak, and birch produced higher pollen concentrations between April and May. Japanese cedar and common sorrel produced higher concentrations of pollens on Jeju, which is isolated from the mainland of the Korean Peninsula.

In terms of regional variation, alder, birch, cedars, and oriental thuja began to produce pollens in February, after which their concentrations increased. In Seoul and Guri, pine pollen concentration was very high in May, when the concentrations of oak and birch pollen were also high. Common ragweed appeared at 5/m³/day in the middle of July; its concentration was highest in the middle of September. Japanese hop pollen levels were highest between the middle of August and the end of September. Mugwort pollen appeared in the middle of August and its concentration increased up until early September (Fig. 5). Birch pollen appeared earlier (early February) in Kangneung than in the other areas; pine pollen also appeared at higher concentrations in Kangneung than in other areas (Fig. 6). However there is not enough data to make a pollen calendar in Chuncheon because pollens were collected since 2010. In the Daegu area, oriental thuja, alder, and juniper produced a large amount of pollens in March. Pine produced a large amount of pollen between the middle of April and the end of May. Weed pollen concentrations were higher in September as in the other areas, and mugwort pollen was produced earlier than common ragweed pollen (Fig. 7). In Busan, flowering occurred relatively early, and alder and oriental thuja pollens appeared here before they did in other areas (early February) (Fig. 8). However there is not enough data to make a pollen calendar in Ulsan because pollens were collected since 2010. In Daejeon, birch and alder produced pollens earlier than in the other areas. Pine pollen was present at high levels between the beginning of April and the end of May (Fig. 9). In the Kwangju, oriental thuja and hazelnut pollens appeared in early February (Fig. 10). But there is not enough data to make a calendar in Jeonju because pollens were collected since 2009. In the Jeju area, Japanese cedar pollen appeared at higher concentrations than in other areas in March and peaked between the middle of February and March. Additionally, common sorrel pollen appeared at high levels between May and June. These results reflect the characteristics of Jeju (Fig. 11).

In conclusion, the pollen calendar in South Korea made between 1997 and 2002 does not reflect current pollen distribution and concentrations, because these have been altered by changes in the weather and environment. Therefore, updated information on pollen distribution and concentration in each region should be provided to the general public and allergy patients through a website in order to manage and prevent pollinosis.