I have read with interest the many helpful responses to this question, but only a few have mentioned quantitative performance testing as imperative to ensuring the containment robustness of ventilated enclosures. As a former member of the ASHRAE 110 committee, and a current member of the ANSI Z9.5 committee, I would be the first to admit that tracer gas testing is not the "be all and end all" of containment assessment, but to date it is the most accurate and reliable assessment tool that I have come across.

At the company where I work we have conducted literally thousands of tracer gas tests on fume hoods and other ventilated enclosures over the last 15+ years. While the majority of those tests have been conducted at approximately 100 fpm, some have been as low as 60 fpm and as high as 150 fpm. Across this spectrum, if you plot face velocity vs. tracer gas containment, you find that the correlation coefficient is extremely low, indicating that face velocity is a poor predictor of containment capability. Per the ASHRAE 110 method we also conduct high and low volume smoke tests, and we have seen many cases where both face velocity and smoke look good, but the hood fails tracer gas testing.

The moral is that, regardless of face velocity,every fume hood and many other ventilated enclosures must be tracer gas tested after installation in order to ensure adequate containment. Just face velocity and smoke testing are not adequate, and even tracer gas testing at the factory and/or random tracer gas testing in the field is not adequate. Identical hoods in different locations will not necessarily perform the same. In fact, we have seen identical enclosures sitting side by side and one will pass and the other will fail. Yes tracer gas testing costs more money, but in the long run it is a good investment.