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We are sensitive to the appropriate treatment of data 

reported as detection limit values in the development of MACT 

floors and emissions limits. We believe that the statistical 

probability procedures applied in calculating the floor or an 

emissions limit inherently and reasonably account for emissions 

data variability including measurement imprecision when the 

database represents multiple tests from multiple emissions units 

for which all of the data are measured significantly above the 

method detection level. That is less true when the database 

includes emissions occurring below method detection capabilities 

regardless of how those data are reported. 

The EPA’s guidance to respondents for reporting pollutant 

emissions used to support the data collection specified the 

criteria for determining test-specific method detection levels. 

Those criteria ensure that there is only about a 1 percent 

probability of an error in deciding that the pollutant measured 

at the method detection level is present when in fact it was 

absent. (Reference: ReMAP: PHASE 1, Precision of Manual Stack 

Emission Measurements; American Society of Mechanical Engineers, 

Research Committee on Industrial and Municipal Waste, February 

2001.) Such a probability is also called a false positive or the 

alpha, Type I, error. This means specifically that for a 

normally distributed set of measurement data, 99 out of 100 
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single measurements will fall within ±2.54 x standard deviation 

of the true concentration. The anticipated range for the average 

of repeated measurements comes progressively closer to the true 

concentration. More precisely, the anticipated range varies 

inversely with the square root of the number of measurements. 

Thus, for a known standard deviation (SD) of anticipated single 

measurements, the anticipated range for 99 out of 100 future 

triplicate measurements will fall within ± 2.54 SD/√3 of the 

true concentration. This relationship translates to an expected 

measurement imprecision for an emissions value occurring at or 

near the method detection level of about 40 to 50 percent.

By assuming a similar distribution of measurements across a 

range of values and increasing the mean value to a higher 

representative value (e.g., 3 times method detection level or 

3xMDL), we can estimate measurement imprecision at other levels. 

For a measurement at 3xMDL, the estimated measurement 

imprecision for a three test run average value would be on the 

order 10 to 20 percent. This is about the same measurement 

imprecision as found for Methods 23 and 29 indicated in the ASME

ReMAP study for the sample volumes prescribed in the final rule 

(e.g., 4 to 6 dscm) for multiple tests.

Instead of the method detection limit, analytical 

laboratories often report a value above the method detection 
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limit that represents the laboratory’s perceived confidence in 

the quality of the value. This independently adjusted value is 

expressed differently by various laboratories and is called 

limit of quantitation (LOQ), practical quantitation limit (PQL), 

or reporting limit (RL). In many cases, the LOQ, PQL, or RL is 

simply a multiplication of the method detection limit. Commonly 

used multipliers range from 3 to 10. Because these values 

reflect individual laboratories’ perceived confidence, and, 

therefore, could be viewed as arbitrary, we decline to adopt the 

LOQ, PQL, or RL in the database for calculating floor values 

because such approaches in our view would inappropriately

inflate the MACT floor standards. Our alternative to those 

inconsistent approaches is discussed below.

We recognize that an emissions limit based on a small 

subset of data or data from a single source may be significantly 

different than the actual method detection levels achieved by 

the best performing testing companies in practice. This fact, 

combined with the low levels of emissions measured from many of 

the best performing units, led us to develop a procedure 

intended to account for the contribution of measurement 

imprecision to data variability in establishing effective 

emissions limits. That procedure is for identifying a 

representative method detection level (RDL). 
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For the total organic HAP emissions data, we applied the 

following procedure for determining an RDL for methods used in 

measuring organic HAPs. We determined method detection 

capabilities for Method 320 and Method 18 as appropriate for the 

various compounds (e.g., Method 320 for aldehydes, Method 18 for 

arenes). We believe that this approach is representative of 

procedures practiced by the better performing testing companies 

and laboratories using the reasonably applicable and sensitive 

analytical procedures. We determined for each of the organic 

HAPs the expected method detection level for the respective 

method based on internal experience and method capabilities

reported by testing companies. With these reported values, we 

identified the resulting mean of the method detection levels as 

the representative detection level (RDL) because it is

characteristic of accepted source emissions measurement 

performance. The following table summarizes those findings.

Pollutant

Method used RDL

ppmv, as measured

RDL

ppmv @7% O2 dry

Formaldehyde Method 320 0.15 0.23

Acetaldehyde “ 1.50 2.34

Benzene

Method 18 

(adsorbent tube) 0.04 0.08

Toluene “ 0.04 0.08

Styrene “ 0.04 0.08

Xylenes 

(m,p,o)

“

0.12 0.24

Napthalene “ 0.03 0.06

The second step in the process is to calculate 3xRDL for 
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the sum or total of the organic HAPs to compare with the 

emissions limit in the final rule. This step is similar to what 

we have used for other NESHAP including the proposed Portland 

Cement rule. As outlined above, we use the multiplication factor 

of 3 to reduce the imprecision of the analytical method until 

the imprecision in the field sampling reflects the relative 

method precision as estimated by the ASME ReMAP study. That 

study indicates that such relative imprecision remains a 

constant 10 to 20 percent over the range of the method. For 

assessing the calculated floor results relative to measurement 

method capabilities, if 3xRDL were less than the calculated 

floor or emissions limit (e.g., calculated from the upper 

predictive limit, UPL), we would conclude that measurement 

variability was adequately addressed with the initial floor 

calculation. The calculated floor or emissions limit would need 

no adjustment. If, on the other hand, the value equal to 3xRDL 

were greater than the UPL, we would conclude that the calculated 

floor or emissions limit did not account entirely for 

measurement variability. Where such was the case, we would 

substitute the value equal to 3xRDL for the calculated floor or 

emissions limit. This would result in an emissions limit where 

the method would produce measurement accuracy on the order of 10 

to 20 percent similar to other EPA test methods and the results 
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found in the ASME ReMAP study.

We summed the RDLs adjusted for dilution and moisture as 

shown in the table above. We determined 3xRDL for the sum of the 

adjusted pollutants to be 11.2 ppmv @ 7 percent O2, dry. This 

value is greater than the final 9 ppmv @ 7 percent O2, dry, in 

the final rule. This result means that we would propose to 

adjust the total organic HAP limit to 12 ppmv @ 7 percent O2, 

dry. At this level, we believe that currently available 

emissions testing procedures and technologies provide the 

measurement certainty sufficient for sources to demonstrate 

compliance at the levels of the revised emissions limit.