1. V. Pavlov, Applied Statistics, 2013, (Prespres, Sofia, 2013), 160p, ISBN 978-954-90443-6-2. [in Bulgarian]
2. V. Pavlov, V. Mihova, Applied Statistics with SPSS, (Avangard Print, Bulgaria, 2016), 192p, ISBN 978-954-337-308-6. [in Bulgarian]
3. Normative acts for the atmosphere air, Ministry of Environment and Water of Republic of Bulgaria, 1999, available at https://www.moew.government.bg/bg/vuzduh/kachestvo-na-atmosferniya-vuzduh/ normativni-aktove/.
4. Register of the towns and villages in Ruse region, Wikipedia, 2022, available at https://bg.wikipedia.org/wiki/%D0%A0%D1%83%D1%81%D0%B5_(%D0%BE%D0%B1%D0%B B%D0%B0%D1%81%D1%82).
5. Criteria for air pollutants, Britannica, 2022, available at https://www.britannica.com/search?query=criteria+air+pollutants+.