Which of the following is true about the forty percent rule?

The forty percent rule, which relates to the use of enriched air nitrox (EAN) in diving, indicates that gas mixtures containing up to 40% oxygen can be used without the necessity for special equipment preparation, modification, or maintenance. This allows divers to utilize EAN 32 or EAN 36, for example, without upgrading their equipment, making it widely accessible for recreational divers who want to enhance their underwater experience by extending their no-decompression limits.

This rule simplifies procedures for divers, as standard recreational diving equipment is generally compatible with these gas mixtures. It also highlights the safety and practical aspects of using these specific mixtures, which are popular among divers looking to optimize their air supply for longer dives and reduced nitrogen absorption, thereby minimizing the risk of decompression sickness.

In contrast, gas mixtures that exceed 40% oxygen typically require additional considerations such as specific training for handling high-oxygen environments, specialized gas tank preparations, and possibly different equipment to mitigate risks associated with oxygen toxicity. Thus, the statement that equipment for gas mixtures up to EAN 40 does not require special preparation is accurate and reflects the principle behind the forty percent rule.