Acetylene

Endpoint summary

Key value for chemical safety assessment

Bioaccumulation potential:

no bioaccumulation potential

Additional information

Physiological effects related to acetylene absorption following inhalation have long been known and the gas was used as an anaesthetic for many years. More recently, acetylene has been used to measure cardiac output using a method referred to as the acetylene re-breathing technique; this depends on measurements of the wash-out rate from blood following absorption. During re-breathing experiments using an inhaled concentration of 800 ppm (856 mg/m3), the concentration of acetylene in inhaled and exhaled air were very similar after 15 minutes indicating rapid equilibration (ten Berge, 2009). The human blood:air partition coefficient for acetylene is 0.833 (Jibelian et al., 1981). The biotransformation of acetylene has been summarised by ten Berge (2009) (see Appendix 1 to the CSR), key points are identified below.  The possibility of a reaction between acetylene and cytochrome P-450 was initially identified by White (1978) who demonstrated the in vitro formation of a water soluble product. Formation of adducts during oxidation by cytochrome P-450 results in inactivation of the enzyme; this may be a reversible effect. Adducts have been isolated and shown to be formed by alkylation of the prosthetic haem (Ortiz de Montellano & Kunze, 1980; Ortiz de Montellano et al., 1982). The loss of cytochrome P-450 activity due to exposure to 2500ppm acetylene for 8 hours has been calculated to be approximately 0.7% per day; the renewal rate in humans for the relevant P-450 enzymes is approximately 30% per day. An alternative metabolic pathway for acetylene involving oxidation of the triple bond can result in a ketene (ethenone); this product is rapidly hydrolysed to acetic acid.  ten Berge (2009) estimated the relative rates of the two pathways and concluded that ketene formation is much faster (by about 3300 fold); furthermore, the quantity of acetic acid produced during exposure to 2500ppm acetylene is not sufficient to result in an effect on the pH of tissues. Biotransformation of acetylene was estimated to account for approximately 1% of the inhaled amount. No data exist for dermal or oral absorption, acetylene is a gas at room temperature. Citations ten Berge, W (2009). Scientific reasoning for exempting in vivo mammalian inhalation toxicity studies on acetylene as part of the IUCLID5-file.