The levels of Ni and Co
(8.0 and 3.0 ppm; respectively) were slightly higher in urban dust
than at the industrial (6.6 and 2.0) and at the rural sites (5.0 and
1.7), and this suggests that the major contributors of Ni in the environment are industrial smelters, beside its frequent presence
in manure (Zheng et al., 2009; Abdullah et al., 2015).