Table 6.

Recommendations for contact time using halogen disinfection in the field.

Concentration of halogen	Contact time (min) at various water temperatures
	5°C (41 °F)	15°C (59°F)	30°C (86°F)
2 ppm	240	180	60
4 ppm	180	60	45
8 ppm	60	30	15

Concentration and contact time are based on the most resistant target organism, which is the Giardia cyst. These are well beyond the time needed to kill bacteria and viruses. These contact times have been extended from the usual recommendations in cold water to account for the extended inactivation time required in very cold water and for the uncertainty of residual concentration.