Direct Reading Gas Detectors for toxic killers - Robert E. Henderson

Recent disasters point to the importance of identifying specific toxic hazards in the workplace, and doing something about them.

On 23rd December 2003 a gas well blowout near the city of Chongqing in central China released a deadly mixture of natural gas and hydrogen sulphide. The toxic cloud killed 243, caused the hospitalization and treatment of more than 9,000, and the evacuation of more than 60,000 nearby residents. Only two of those killed were gas field employees. The rest were residents of the surrounding area. Although official figures report 243 deaths, many survivors say that the true number exceeds 300.

On 16 April 2004 five large chlorine tanks at a chemical plant in central China exploded, killing nine workers, and leading to the evacuation of over 150,000 nearby residents. The tanks had been in the process of being drained when a sudden rise in temperature led to the explosion. Three remaining chlorine gassifier tanks had to be destroyed by military armourpiercing anti-tank weapons. Over 3,000 policemen, 650 armed police soldiers and 160 firemen were involved in the emergency response and detonation of the remaining tanks.

The Organization of Economic Cooperation and Development (OECD) produce a list of High Production Volume (HPV) Chemicals. According to the OECD, over 5,000 toxic chemicals are produced or imported in volumes in excess of 1,000 tons per year in the North America. On average, there are about 400 major accidents per year in the United States and Canada involving these chemicals.

Many of the most common toxic gases, including carbon monoxide, hydrogen sulphide, sulphur dioxide, chlorine, ammonia, cyanide, ethylene oxide, nitric oxide, nitrogen dioxide and chlorine dioxide can be measured by means of compact, substance-specific electrochemical sensors. Gas that enters the sensor undergoes an electrochemical reaction that causes a change in the electrical output of the sensor. The difference in the electrical output is proportional to the amount of gas present. The sensors are designed to minimize the effects of interfering contaminants on readings. Electrochemical sensors are compact, require little power, exhibit excellent linearity and repeatability, and generally have long life span.

New sensor technologies such as miniaturized photoionization detectors (PIDs) for volatile organic contaminant (VOC) measurement, and non-dispersive infrared (NDIR) sensors for CO2 have increased the number of toxic gases that can be measured by means of compact, portable gas detectors. As exposure limits continue to drop, atmospheric monitoring programs increasingly need to include direct quantifiable measurement for many additional toxic substances.