Toxic emissions threshold limit values

A local exhaust is generally preferred over a dilution ventilation system for health hazard control because a local exhaust system removes the contaminants directly fi om the source, whereas dilution ventilation merely mixes the contaminant with uncontaminated air to reduce the contaminant concentration. Dilution ventilation may be acceptable when the contaminant concentration has a low toxicity or a threshold limit value (TLV) > 100 ppm, and the rate of contaminant emission is constant and low enough that the quantity of required dilution air is not prohibitively large. However, dilution ventilation generally is not acceptable when the TLV is less than 100 ppm. 

Carbon monoxide. Carbon monoxide is a colorless, odorless gas, liquefying at -191.5°. It is readily available, at pressures of up to about 100 bar, in a variety of cylinder sizes, so that experiments can be run at 80 bar without a compressor. The acute toxicity of carbon monoxide reflects its high affinity for hemoglobin. It is stated" that 400-500 ppm of carbon monoxide in air can be breathed for an hour without appreciable effect, while levels above 1000 ppm are dangerous, and at more than 4000 ppm it causes death within an hour. The previously cited reference gives a Threshold Limit Value of 50 ppm. Carbon monoxide must always be used in an efficient fume cupboard, keeping emissions into the laboratory atmo-... 

During the past 15 years formaldehyde exposures and emission limits have been significantly lowered. Occuptional threshold limits are now 1.0 ppm or lower in most countries, and actual industrial exposures are almost always half of this value or less. Indoor air standards of 0.1 ppm are now contemplated in several nations, following established procedures for correlating occupational levels of toxic chemicals with ambient air levels. Furthermore, emission standards for UF-bonded wood products have been developed that allow the prediction of formaldehyde levels under various product use conditions before formaldehyde emitting products are installed. 

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