When controls are used to reduce the level of atmospheric contaminants to acceptable levels, testing must be conducted to

ensure that the atmosphere remains safe

Explanation:

As a general industry standard, the construction safety standard requires testing the atmosphere and verifying that it is safe before someone enters the confined space. Specifically, the standard states the following: Before an employee enters the space, the internal atmosphere must be tested with a calibrated direct reading instrument, in order to detect the content of oxygen, flammable vapors and gases and possible toxic air pollutants , in that order. Employees entering the space, or their authorized representative.