In the early fabrication of fibers (mid-1960s to early 1970s), impurities by absorption was the major reason for high attenuation of the optical signal. An extremely high level of purity against some elements is necessary. For example, two parts per billion (ppb) of cobalt can induce a 10 dB/km loss; 20 ppb of nickel, iron or chromium, or 50 ppb of copper, or even 100 ppb of manganese or vanadium can each induce 10 dB/km loss through the fiber. In 1970, the quality control of the fiber manufacture process was poor enough to induce a 20 dB/km fiber. In 1972, the loss was reduced to about 4 dB/km and fiber for communication became scientifically and economically feasible for longer distance trunk applications.