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I would like to comment on Section 37.4 of the revised ANSI
Standard 867 for testing air cleaners.  This section allows
monitoring for 8 hours instead of 24 hours if a steady state has
been reached.  "Steady state" is defined as one in which the slope
between hours 7 and 8 is not positive.  This definition is
defective.
 
Let us assume that a true steady-state has been achieved.  Then
half of the measured slopes will show a small positive value and
half will show a small negative value.  
 
According to the defective definition, half of all cases that
indeed achieved a steady state after 7 hours would be wastefully
required to complete 24 hours of testing.
 
The economic impact of this depends on the fraction of cases that
do reach a steady state in 7-8 hours, and on the difference in
cost involved in monitoring for 24 hours instead of 8.
 
The definition of the steady state should allow de minimus
positive slopes to be proof of a steady state.  The value of this
positive cutoff can be determined as a function of the allowed
precision of the measurement method (2%) coupled with observations
of the slopes obtained by a sufficient number of Monte Carlo runs
of 60 1-minute averages on a constant concentration subject to
random 2% errors.   

I am aware that the ARB can not themselves change the ANSI
Standard but in view of the possible economic consequences of the
ARB regulation based on this standard, perhaps the Board can bring
this to the attention of ANSI.