## 1 Introduction  
Climate changes and drought periods mean that alternative sources of water supplies are required. Rainwater harvesting (RWH) is an old practice [1], commonly used in arid and semi-arid regions [2; 3]. In the countries that struggle with drought, rainwater is collected for potable purposes primarily. However, non-potable applications are practiced also, like washing, cleaning, toilet flushing, or plant watering [4; 5]. Rainwater harvesting is still rarely practiced in urban regions of Poland [6], but as water resources in Poland are almost the lowest in Europe [7; 8; 9], RWH systems may become a necessity. Lately, roof runoff harvesting has also been promoted, as it can reduce excess runoff and consumption of potable water at the same time [10; 11]. Although, roof runoff seems to be much cleaner than stormwater runoff [12; 13] it is not completely free form different types of contaminants, like heavy metals, organic pollutants, bacteria, and fungi [14; 15; 16; 17]. The quality of harvested rainwater depends on roofing materials, climatic conditions, and atmospheric pollution [18]. According to [PERSON] [17], the urban runoff and the combined sewage overflows events occurring during wet weather are the major sources of water pollution. Moreover, the roof runoff may cause more than 80% of the load of heavy metals in the combined sewage system during wet weather [17]. There is no classification designed to determine rainwater pollution in terms of its potential use. Existing classifications for potable water are being used as they are the most strict according to water quality. Thus, in this study ecological and health risk assessment methods were also used to determine the reliable threats arising from pollutant contents in urban roof runoff for its potential non-potable usage. Contamination of roof runoff might be an issue, especially in urban areas where the air quality is often poor due to road traffic effects and local pollution-emission sources [19; 20; 21]. As air humidity and wet deposition decrease the contents of pollutants in ambient air [19], they may further cause contamination of rainfall [22; 23]. Air and precipitation pollutant concentrations may be augmented by various groups of contaminants that negatively affect human health. According to the European Environment Agency, the following are the most harmful air pollutants affecting human health in Europe: suspended in the air particulate matter (PM), gaseous ozone (O\({}_{3}\)) and nitrogen dioxide (NO\({}_{2}\)), volatile liquid benzene (C\({}_{6}\)H\({}_{6}\)), and bound with PM particles benzo[a]pyrene (BaP) and heavy metals. The major sources of PM in the urban areas in Europe include fuel combustion during the heating season, household heating, and fuel combustion by road vehicles [24]. Long-term exposure to PM concentrations can shorten life expectancy [25]. Heavy metals, in particular: As, Cd, Co, Cr(VI), Ni, and Pb may cause the following health effects: cancer, cardiovascular diseases, or metabolic diseases, such as obesity and diabetes mellitus [26; 27]. Organic pollutants, like BaP, bounded with PM particles, are primarily linked with carcinogenic effects in humans, but also show strong mutagenicity [25]. Airborne microorganisms like viruses (e.g., Influenza A and Measles/Rubeola, or Norwalk-like viruses, severe acute respiratory syndrome-associated coronaviruses, Sin Nombre viruses, Varicella-zoster viruses, or Variola viruses), bacteria (e.g., _Bacillus anthracis, Francisella tularensis, Legionella pneumophila, Mycobacterium tuberculosis_, or _Yersinius pestis_), and fungi (e.g., _Aspergillus fumigatus, Aspergillus versicolor, Blastomyces dermatitis, Chaetomium_ sp., _Cocchioides immitis, Cryptoccus neoformans, Histoplasma capsulatum, Penticillium_ sp., _Stachybortys charatum_, and _Trichoderma_ sp.) are also associated with potential adverse effects on human health [28; 29]. Such types of bioaerosols are washed out from the air and roof surfaces, gutters and downpipes, causing roof runoff contamination.