VOC

Volatile Organic Compounds, VOCs

Organic compounds in general are those that contain carbon and hydrogen, and originate from life forms. Typical compounds are petroleum, toluene, benzene, phenolics, ketones such as methyl isobutyl ketone (MIBK), aldehydes, and esters such as methyl methacrylate. Quite a number of these have a strong odour. Some are quite toxic while others have a low toxicity. Organic compounds with smaller molecules are usually volatile, that is their vapour fi lls the air space around the solid or liquid organic compound. This vapour may be explosive, or it may provide a route for inhalation or skin contact with resultant adverse health effects.

Adverse health effects can be acute, that is observable immediately, or chronic, that is long term. Consequently exposure guidelines usually contain a combination of short term and longer term measures. The short term is expressed as a Short Term Exposure Limit (STEL), measured over 10 or 15 minutes, while the longer term is expressed as an 8 hour Time Weighted Average (TWA). Exposure standards for most VOCs are given in mg/m3 of air. The 8 hour TWA is derived by assuming that a person works 8 hours a day, 5 days a week. If someone works a different schedule the allowable standard must be recalculated. Assumptions of what is still a quite common working week can result in the allowable TWA being reduced to near half the 8 hour 5 day TWA.

There are several methods for measuring VOCs. Some are more sensititive or compound specifi c than others, and naturally the cost of the measurement depends on how accurate or specific the method is.

Measurement

A common method is to adsorb any volatile materials onto a selected medium contained in a tube. This is done by drawing a measured volume of air through the tube using a calibrated constant volume air sampling pump. The air flow rate must be suffi ciently slow so that the VOC has time to adsorb onto the medium in the tube. The tube’s contents are then analysed for either the specifi c compound of interest or for a scan of VOCs in general. Each VOC can be individually identifi ed, quantified and the calculated concentration in air compared with the exposure standard. To check that all the VOCs in the sampled air have been adsorbed, the contents of the tube are divided into two sections, front and back, and each section is analysed separately. If VOC is detected in the rear section then there has been breakthrough and the results may understate the real amount of VOC in the air. This method works well for an 8 hour TWA but may not have suffi cient sensitivity for detection of VOC from a sampling time of 15 minutes to determine the STEL. Other related methods involve a number of variations such as sampling and analysing cylinders of air. A simple method suitable for measuring short term concentration is to draw air through a colorimetric tube.

A different approach is to use an instrument fi tted with a Photo Ionisation Detector, PID. This is akin to an electronic nose. The instrument detects over 300 compounds, but it does not respond equally to all of them, so it has limited value in assessing a mixture of VOCs. If the air contains only one VOC then a factor can be applied to calculate the concentration of that particular compound. The PID is usually calibrated to isobutylene and the correction factors are usually given in relation to the response to isobutylene. Most PIDs are fi tted with a lamp of a standard electron voltage, but for assessing semi volatile organic compounds, that is heavier compounds, a different lamp is required. An advantage of a PID is that the reading is real time, and the instrument can be set to datalog changes over time.

Download the Volatile Organic Compounds Information Sheet (PDF)