Joint International Conference on Long-term Experiments ...

output of the monitor was plotted against HCHO vapor levels obtained by chemical
analysis, a power-law relationship was obtained. Such a relationship is particularly useful,
since good sensitivity was achieved at the lower vapor levels and the total fumigation range
was covered on the same scale.
Relationship between temperature and equilibrium vapor concentration. The
relationship between temperature and the equilibrium vapor concentration is shown in. For
normal air-conditioned laboratories at 20 o C, the equilibrium vapor concentration is
approximately 2000 ,ug of HCHO per liter. HCHO vapor in excess of the equilibrium vapor
concentration will recondense on the coldest surfaces, producing a paraformaldehyde film
over surfaces within the room. This film slowly evaporates, producing an unpleasant odor.
HCHO levels after fumigation. The persistence of HCHO vapor after completion of the
fumigation and restoration of ventilation is a significant problem, especially in high-security
laboratories. The extent of the problem can be seen from chart recordings from the
electronic monitor during fumigation of a high- and a low-security area.
DISCUSSION
The data presented above demonstrate several advantages of the HCHO generator over the
Formalin-permanganate method in HCHO fumigation. Rapidly falling vapor levels in the
permanganate method are due, in part, to the formation of water condensate on cold surfaces
which, because of the high partition of coefficient of HCHO in water , rapidly takes up the
HCHO vapor. High-localized HCHO levels generated by rapid vaporization in this method
also lead to excessive condensation of paraformaldehyde.
From the standpoints of safety, convenience, and effectiveness, the HCHO generator
method was more successful, particularly where large areas were to be fumigated. It is not
necessary to rapidly leave the area after addition of Formalin. Instead, it is only necessary to
adjust a timer well in advance of the fumigation, and any necessary sealing of doors and
other outlets can then proceed at leisure and without the discomfort of leaking vapor. The
importance of adequate sealing is illustrated by the rapid loss of fumigant from a lowsecurity
area. Slow diffusion of vapor into closed cupboards was demonstrated in procedure
1. Cupboards, drawers, and other such fittings in the room must be opened to allow
adequate penetration of fumigant.
The value of maintenance of a uniform temperature was particularly apparent at a
fumigant concentration of 1.500, ug of HCHO per liter. At this level, the restoration of
ventilation has rapidly removed all traces of the fumigant down to the level of human
detection, commonly accepted as 1 ug/liter. This level complies with most of the
international standards currently in use.
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