Half Time (Tissue Half Time) in Scuba Diving


Half time, also known as tissue half time, is a critical concept in scuba diving, specifically in relation to decompression theory. It pertains to the time taken for a dissolved gas in a tissue, such as nitrogen, to equilibrate to a new pressure or to reach full saturation at a new pressure. Understanding this principle is essential for divers to manage their decompression schedules and prevent decompression sickness (DCS). This article delves into the concept of half time, its role in scuba diving, and its connection to decompression tables and dive computers.

Decompression Theory and Half Time

Decompression theory aims to understand the behavior of gases in the body under pressure changes during scuba diving. As a diver descends, the pressure of the surrounding water increases, causing the partial pressures of the gases in the breathing mixture to rise. Consequently, the body tissues absorb more inert gas (e.g., nitrogen) from the blood. Upon ascent, the pressure decreases, and the excess inert gas must be released gradually to prevent the formation of gas bubbles in the tissues, which could lead to DCS.

Half time is a measure of how quickly tissues absorb and release gas. It represents the time it takes for a tissue to absorb or release 50% of the difference between its current and final gas concentrations under a new pressure. In scuba diving, nitrogen is the primary inert gas of concern, as it constitutes approximately 79% of the air we breathe. The rate at which nitrogen is absorbed or released by tissues depends on factors such as tissue type, blood flow, and pressure changes.

Tissue Compartments and Half Times

The human body comprises various tissues with different absorption and release rates for nitrogen. To simplify the process of tracking nitrogen levels, decompression models divide the body into hypothetical tissue compartments, each with a specific half time. These compartments are grouped into three categories based on their half times:

  1. Fast tissues: These tissues have short half times, ranging from 2.5 to 10 minutes. They include the central nervous system, blood, and skin. Fast tissues absorb and release nitrogen quickly, making them particularly vulnerable to rapid pressure changes.
  2. Medium tissues: These tissues have half times ranging from 20 to 40 minutes. They include muscles and tendons. Medium tissues have moderate absorption and release rates, making them relevant for most recreational dive profiles.
  3. Slow tissues: These tissues have long half times, ranging from 60 to 120 minutes or more. They include bones, cartilage, and ligaments. Slow tissues take longer to absorb and release nitrogen and are less susceptible to rapid pressure changes.

Decompression Tables and Dive Computers

Half times are central to the development of decompression tables and algorithms used in dive computers. These tools help divers plan their ascent rates and decompression stops to ensure the safe release of excess nitrogen. Decompression tables, such as the U.S. Navy tables or the Bühlmann tables, provide guidelines on ascent rates and stop times based on dive depth, duration, and previous dives.

Dive computers, on the other hand, use algorithms to calculate a diver’s real-time nitrogen levels in various tissue compartments. These algorithms take into account factors such as depth, dive duration, and previous dives to provide personalized ascent rates and decompression stops. Dive computers often incorporate different decompression models, allowing divers to choose the one that best suits their diving style and risk tolerance.

Preventing Decompression Sickness

Understanding half times and adhering to recommended ascent rates and decompression stops can help prevent DCS. However, it is essential to note that individual factors, such as age, fitness, and health, can affect a diver

‘s susceptibility to DCS. Therefore, divers should also consider their unique circumstances when planning dives.

To further minimize the risk of DCS, divers can implement the following precautions:

  1. Proper dive planning: Careful dive planning, taking into account depth, duration, and previous dives, is critical in managing nitrogen absorption and release. Using decompression tables or dive computers can help divers plan their ascent rates and decompression stops effectively.
  2. Gradual ascent: Adhering to a slow and controlled ascent rate allows the body to release excess nitrogen gradually. A general guideline is to ascend no faster than 30 feet (9 meters) per minute.
  3. Safety stops: Performing a safety stop at 15-20 feet (5-6 meters) for 3-5 minutes at the end of each dive provides additional time for tissues to release excess nitrogen, especially for dives deeper than 30 feet (9 meters).
  4. Surface intervals: Allowing ample time between dives for the body to off-gas residual nitrogen can reduce the risk of DCS. Surface intervals should be determined based on previous dives, using decompression tables or dive computers as a guide.
  5. Hydration and fitness: Staying well-hydrated and maintaining a good level of physical fitness can improve a diver’s ability to off-gas nitrogen efficiently.
  6. Conservative diving: Choosing a conservative dive profile, which allows for longer decompression stops or shallower depths, can help reduce the risk of DCS. Some dive computers offer a “conservative” setting, which adjusts the algorithm to provide more conservative decompression schedules.
  7. Avoiding alcohol and drugs: Consuming alcohol or drugs before or during a dive can impair judgment and affect the body’s ability to off-gas nitrogen, increasing the risk of DCS.
  8. Monitoring post-dive symptoms: Divers should be vigilant for signs and symptoms of DCS after a dive, such as joint pain, skin rash, dizziness, or fatigue. If DCS is suspected, seek immediate medical attention.


Half time, or tissue half time, is a critical concept in scuba diving, as it influences the rate at which tissues absorb and release nitrogen under pressure changes. By understanding half times and applying this knowledge to decompression planning, divers can manage their ascent rates and decompression stops, reducing the risk of decompression sickness. Utilizing decompression tables, dive computers, and following best practices for safe diving will contribute to a more enjoyable and secure scuba diving experience.