For the purpose of this section, the following definitions apply:

Flight segment means scheduled nonstop flight time between two airports.

Sea level equivalent refers to conditions of 25 °C and 760 millimeters of mercury pressure.

Except as provided in paragraphs (d) and (e) of this section, no certificate holder may operate an airplane above the following flight levels unless it is successfully demonstrated to the Administrator that the concentration of ozone inside the cabin will not exceed—

For flight above flight level 320, 0.25 parts per million by volume, sea level equivalent, at any time above that flight level; and

For flight above flight level 270, 0.1 parts per million by volume, sea level equivalent, time-weighted average for each flight segment that exceeds 4 hours and includes flight above that flight level. (For this purpose, the amount of ozone below flight level 180 is considered to be zero.)

Compliance with this section must be shown by analysis or tests, based on either airplane operational procedures and performance limitations or the certificate holder's operations. The analysis or tests must show either of the following:

Atmospheric ozone statistics indicate, with a statistical confidence of at least 84%, that at the altitudes and locations at which the airplane will be operated cabin ozone concentrations will not exceed the limits prescribed by paragraph (b) of this section.

The airplane ventilation system including any ozone control equipment, will maintain cabin ozone concentrations at or below the limits prescribed by paragraph (b) of this section.

A certificate holder may obtain an authorization to deviate from the requirements of paragraph (b) of this section, by an amendment to its operations specifications, if—

It shows that due to circumstances beyond its control or to unreasonable economic burden it cannot comply for a specified period of time; and

It has submitted a plan acceptable to the Administrator to effect compliance to the extent possible.

A certificate holder need not comply with the requirements of paragraph (b) of this section for an aircraft—

When the only persons carried are flight crewmembers and persons listed in § 121.583;

If the aircraft is scheduled for retirement before January 1, 1985; or

If the aircraft is scheduled for re-engining under the provisions of subpart E of part 91, until it is re-engined.