When the air pressure in any tunnel, caisson, compartment, or place in which persons are employed exceeds normal, but does not exceed fifty pounds, the maximum number of hours which, in any twenty-four hours, a person may be employed or permitted to work or remain therein, shall be as hereinafter stated. In every case the maximum number of hours shall be divided into two working-periods of equal length, and the minimum time interval which shall elapse between such working-periods shall be as hereafter stated.

When the air pressure exceeds normal, but does not exceed twenty-one pounds, number of hours in twenty-four, eight; interval between working-periods, thirty minutes.

When the air pressure exceeds twenty-one but does not exceed thirty pounds, number of hours in twenty-four, six; interval between working-periods, one hour.

When the air pressure exceeds thirty but does not exceed thirty-five pounds, number of hours in twenty-four, four; interval between working-periods, two hours.

When the air pressure exceeds thirty-five but does not exceed forty pounds, number of hours in twenty-four, three; interval between periods, three hours.

When the air pressure exceeds forty but does not exceed forty-five pounds, number of hours in twenty-four, two; interval between working-periods, four hours.

When the air pressure exceeds forty-five but does not exceed fifty pounds, number of hours in twenty-four, one and one-half; interval between working-periods, five hours.

Except in cases of emergency, no person shall be employed, or permitted to work or remain, in any tunnel, caisson, compartment, or place where air pressure exceeds fifty pounds.