At the early stage of exposure (less than one year) the CL of CS and WS are similar at all test sites. After one year-exposure the CL of WS become lower than that of CS, this was expressed clearly at Hanoi and Dong Hoi sites (fig. 1a, 1b), whereas at Phan Rang site the CL of WS and CS are negligible different (fig. 1c). After three year-exposure, the CL of WS become obviously lower in comparison with CS
