years. If the first-choice suicide method is inaccessible, a method substitution may be made which may be less lethal, tending to result in fewer fatal suicide attempts. In an example of the curb cut effect, changes unrelated to suicide have also functioned as suicide method restrictions. Examples of this include changes to align train doors with platforms, switching from coal gas to natural gas in homes, and gun control laws, all of which have reduced suicides despite being intended for a different purpose. Suffocation Suffocation, as a classification of suicide method, includes strangulation and hanging. Suicide by suffocation involves restricting breathing or the amount of oxygen taken in, causing asphyxia and eventually hypoxia. It is not possible to die simply by holding the breath, since a reflex causes the respiratory muscles to contract, forcing an in-breath, and the re-establishment of a normal breathing rhythm. Therefore, inhaling an inert gas such as helium or nitrogen, or a toxic gas such as carbon monoxide, is used to bring about unconsciousness. , organizations supporting a right to die promoted death by helium inhalation, although most cases using this method in the US were people with psychiatric conditions. Hanging Hanging is a common method of suicide. Hanging involves the use of a ligature such as a rope or cord attached to an anchor point with the other end used to form a noose placed around the neck. The cause of death will either be due to strangulation or a broken neck. About half of attempted suicides by hanging result in death. People who favor this method are usually unaware that it is often a "slow, painful, and messy method that [needs] technical knowledge". Hanging is the prevalent means of suicide in impoverished pre-industrial societies, and is more common in rural areas than in urban areas. Hanging was the most common method in traditional Chinese culture, as it was believed that the rage involved in such a death permitted the person's spirit to haunt and torment survivors. In the Chinese culture, suicide by hanging was used as an act of revenge by women and of defiance by powerless officials, who used it as a "final, but unequivocal, way of standing still against and above oppressive authorities". Chinese people would often approach the act ceremonially, including the use of proper attire. Poisoning Suicide by poisoning, also called self-poisoning, is usually classed as a drug overdose when