“…Similar studies have been done by other laboratories and research institutes in Poland, but the scope of these studies was usually limited to individual districts (Amarowicz et al, 1985;Bulinski et al, 1986Bulinski et al, , 1988Chorazy et al, 1987;Kucharski et al, 1989; Gajewska, 1982, 1984;Olejnik et al, 1985;Smoczynski et al, 1984;Szymczak et al, 1984;Zalewski et al, 1989).…”