Table 2.

The toxicity responses (dose responses) to heavy metals as the oral reference doses (RfD) and oral slope factors (SF)

Heavy metal	Oral reference dose (mg/kg/day)	Dermal reference dose (mg/kg/day)	Inhalation reference dose (mg/kg/day)	Slope factor (mg/kg-day)−1
Ni	2 × 10−2	5.4 × 10−3	2.06 × 10−2	0.84
Pb	3.5 × 10−3	5.25 × 10−4	3.52 × 10−3	0.00085
Zn	0.3	6 × 10−2	0.3	n.d
Cu	4 × 10−2	1.2 × 10−2	4.02 × 10−2	n.d
Fe	1.6	n.d	n.d	n.d

n.d: not determined