## 4 Discussion  
The issue of nitrogen dioxide limits, which, if exceeded, have harmful effects on human health, has not been fully resolved yet. However, there has recently been a trend towards lowering these limits. The investigations carried out within this study indicate the occurrence of harmful air pollution (>40 umol/m\({}^{2}\)) in the air of some tourist reception areas in Poland. For mean annual levels, this harmful pollution occurs in: (i) two tourist regions (9.5%): Krakow-Czestochowa and Swietokrzyski; (ii) three national parks (14%): Kampinoski, Ojcowski, and Ujscia Warty; (iii) 11 health resorts (24%): the highest levels being in Swozzowice, Goczalkowice-Zdroj, Konstancin-Jeziorna, Inowroclaw, and Cieplec; (iv) 40 landscape parks (31%); and (v) 12 provincial capitals (75%): the highest levels being in Katowice, Krakow, Warsaw, and Lodz. The extent to which the limits were exceeded varied significantly over time. In January, the limits were exceeded in 53% of national parks, 75% of landscape parks, 57% of tourist regions, 50% of health resorts, and 100% of provincial capitals. In other months, it was clearly smaller:  
1. April: national parks (22%), landscape parks (51%), tourist regions (27%), health resorts (41%), and provincial capitals (94%)
2. July: national parks (0%), landscape parks (2%), tourist regions (0%), health resorts (0%), and provincial capitals (25%)
3. October: national parks (48%), landscape parks (83%), tourist regions (65%), health resorts (56%), and provincial capitals (100%)  
During the summer months, i.e., in periods of the highest tourist traffic, nitrogen dioxide air pollution is basically non-existent. However, for more than half a year, from October to April, the annual threshold value is exceeded in 20% of the national parks, more than half of the landscape parks, 30% of the tourist regions, 40% of the health resorts, and nearly 100% of the provincial capitals. The greatest pollution occurs in tourist reception areas close to major cities. These are areas where tourism and recreation take place all year round (weekend trips). The most polluted national parks in 2019 experienced the following levels of tourist traffic: Wolinski (1.5 million), Kampinoski (1 million), Wielkopolski (1 million), Ojcowski (440,000), Swietokrzyski (120,000), and Ujscie Warty (58,000). The spatial and temporal distribution of NO\({}_{2}\) content is related to meteorological conditions to some extent. The study of these relationships was not the objective of the study, but some relationships can be seen as related to the average wind speeds, for example. Lower pollution occurred in areas with higher speeds--the mountainous areas and the Baltic coast. A more important factor, however, was the location in relation to emission sources. It is higher in the winter and autumn months and spatially concentrated in cities and industrial centres. The COVID-19-related restrictions introduced in Poland in 2020 resulted in a reduction in NO\({}_{2}\) concentrations in the air of tourist reception areas. For February, October, and November, the monthly averages for Poland were 20-30% lower in 2020 than in 2019 (Figure 7). This is due to less car traffic, one of the main sources of pollution. According to [PERSON] et al. (2020), during the lockdown period in Poland, car traffic decreased by 25-50%. In 2021, however, the pollution returned to pre-pandemic levels in Poland. Similar data are provided by studies conducted in other parts of the world ([PERSON] et al., 2019; [PERSON] et al., 2020; [PERSON] et al., 2020; [PERSON] et al., 2020; [PERSON] et al., 2020). Studies of pollution variability are much rarer, but the regularities found there are similar. Periods of lockdown improve the state of the air; however, it worsens much more when tourists return ([PERSON] et al., 2020). Some health resorts with high levels of nitrogen dioxide pollution are identified as facilities where people with pulmonary, ENT, or cardiovascular diseases can be treated.