Extraction of fumes from laser-based materials-processing systems is recognized as essential by both laser users and manufacturers. Until recently, however, little information has been available about the hazards associated with fumes from various different materials and processing methods. This problem is being addressed by the Laser Safety Research Group at Loughborough University of Technology`s Mechanical Engineering department (Loughborough, UK). From its ongoing research into fume extraction as part of the European Eureka 643 program, the group has developed a simple method for characterizing fumes and a specification for fume hazard in terms of one simple parameter, the Normal Air Requirement (NHL).
Fume hazard depends on a number of factors, including material toxicity—determined by the chemical composition of the fume—and the amount of particulate (dust) present that is respirable—known as aerosol. For any given fume constituents there are recommended exposure limits; the Loughborough group has used the Occupational Exposure Standard (OES), for long-term exposure, and the Maximum Exposure Limit (MEL) for highly toxic materials where essentially no exposure level is safe. Both standards are defined by the British Health and Safety Executive (HSE) and are specified in terms of milligrams per second.
The model proposed by the laser safety team at Loughborough is a pragmatic approach to applying highly scientific data in real industrial situations. The fume hazard is calculated using two sets of data. The rate of production of a particular species during processing is combined with the exposure limits noted previously for all major components in the fume. The final output is a single number, the NHL, which can be described as the volume of air required to dilute the fumes to a level at which all the hazardous components are below their exposure limits.
Of the NHLs calculated by the group for several commonly used materials, stainless steel presents the most significant hazard. This is because chromium and nickel—both carcinogenic—are released in processing. Aluminum has a surprisingly low NHL because of its low aerosol emissions.
Nonmetals produce a range of hazards during processing. Kevlar, polycarbonate, polyamide, and some other polymers produce very high aerosol levels—which must be cleared—while some nonmetals also emit toxic compounds, such as benzene. Polyvinyl chloride (PVC) is also an interesting case, given that it is not usually considered an offensive material; it actually poses a significant problem because large amounts of highly corrosive hydrogen chloride can be generated.
More information on other materials is available from the group and more will be added as data are collected from both industrial and academic sources. Dr. Roach, a member of the team, comments, "We believe this approach is a useful starting point for assessing the fume hazard associated with laser processing. It gives both laser manufacturers and users a simple system for determining the significant compounds that are produced from laser processing of materials."