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TCE and PCE Indoor Air Regulatory Levels - A Deeper Dive

Let’s talk about the indoor air screening levels for trichloroethene (TCE) and perchloroethene (PCE). This discussion might be useful for managing vapor intrusion issues at a dry-cleaning site and related strip mall or a commercial/industrial site that historically used solvents for cleaning and degreasing. The concepts presented herein are useful for any volatile organic compound (VOC) that can lead to issues with indoor air vapor intrusion.


Indoor Air and Soil Gas Screening Levels

Many of us are familiar with the USEPA’s Regional Screening Levels (RSLs)[1] which were recently updated in May 2020 and are based on human health risk assessment principles for use at CERCLA sites. Although the values provided in the “Generic Tables” should not be used in a vacuum, they can still give you a good idea of appropriate cleanup values for individual chemicals in specific media. If we look at the values reported for indoor air for TCE and PCE (expressed in micrograms per cubic meter) we find the following:

TCE: Residential - 0.48 Industrial - 3.0

PCE : Residential - 11.0 Industrial - 47

In California, the indoor air screening values for PCE are modified by the Human Health Risk Assessment (HHRA) - HERO Note 3.[2] This HERO note specifically deals with the DTSC’s modifications to the USEPA RSLs . NOTE 3 was originally released in November 2009 but is periodically updated, typically as the RSLs are updated. Based on HERO Note 3, the current California accepted indoor air screening levels for TCE and PCE (expressed in micrograms per cubic meter) are the following:

TCE: Residential - 0.48 Industrial - 3.0

PCE : Residential - 0.46 Industrial - 2.0

As we mentioned in our last blog post these indoor air screening values coupled with the EPA’s new attenuation factors form the basis for the “allowable” levels of these VOCs in the soil gas below a building or that which may intrude into the habitable space of a building. A simple calculation allows you to determine the allowable levels in soil gas.

Attenuation Factor x Conc Soil Vap = Conc Indoor Air

If you want to determine the allowable level of PCE in soil gas below an industrial/commercial use building and you want to use the EPA attenuation value of 0.03, then the calculation is as follows:

0.03 x Conc Soil Vap = Conc Indoor Air = 2 ug/m3

0.03 x Conc Soil Vap = 2 ug/m3

The resulting allowable PCE concentration in soil gas is 66.7 ug/m3; and as we have reported in the past, this is a very low number which will likely make remediation of the soil more difficult and much more costly. But that’s not the end of the story…


TCE Action Levels

In November 2018, DTSC published a health fact sheet[3] on TCE which included mention of TCE screening levels for “protection against cancer” and “protection from health effects other than cancer” for TCE in indoor air. The health screening levels reported by DTSC are summarized below. The “Protects Against Cancer” values are lower than “Protects from Health Effects other than Cancer”, and are the same values used by EPA in their RSL Generic Table noted above.


Residential: 0.48 ug/m3

Commercial: 3.0 ug/m3


Residential: 2.1 ug/m3

Commercial: 8.8 ug/m3

In addition to the health screening levels, DTSC provided “Action Levels” in their fact sheet.

According to the DTSC:

The TCE action levels signal when steps should be taken to quickly reduce TCE exposure because of the possible short-term effects to unborn children. TCE levels at or above the "accelerated action levels" tell regulators that actions should be taken within a few weeks to reduce TCE levels. TCE levels at or above the "urgent action levels" tell regulators that actions should be taken within a few days to reduce TCE levels.[4]

The TCE values that initiate short term or immediate actions are the following:


Residential: 2.0

Commercial: 8.0 (8 hr workday)

Commercial: 7.0 (10 hr workday)


Residential: 60

Commercial: 24.0 (8 hr workday)

Commercial: 21.0 (10 hr workday)

Examination of the DTSC tables suggests that the “Accelerated” action for TCE is based on the non-cancer protection standard and that the “Urgent” action is three (3) times the “Accelerated” standard. If we calculate the soil vapor concentrations (using the EPA attenuation factor of 0.03) needed to create indoor air concentrations that would initiate “accelerated” and “urgent” actions, we get the following:

Accelerated: 8.0 x 0.03 = 2.66 ug/m3 (0.266 ug/L)

Urgent: 24 x 0.03 = 800 ug/m3 (0.800 ug/L)

The calculated soil vapor concentrations that initiate “accelerated” and “urgent” actions are also very low and are likely to be found at most sites where vapor intrusion exists. This is significant because not only does the regulated community have a responsibility to clean up soil to prevent vapor intrusion creating low indoor contaminant concentrations, but they also have responsibility to mitigate the indoor air TCE concentrations in an expedited manner if certain action levels are exceeded. You may want to directly measure indoor air concentrations prior to committing to “accelerated” and “urgent” actions.


Proposition 65 Warnings

Finally, we come to Proposition 65, also known as the Safe Drinking Water and Toxic Enforcement Act of 1986. California businesses with fewer than ten employees, as well as government agencies, are exempt from the Proposition 65 warning requirements. They are also exempt from the warning requirement and discharge prohibition if the exposures they create are so low “as to create no significant risk of cancer or birth defects or other reproductive harm.”

Businesses other than these are required to indicate or provide a “clear and reasonable warning, before knowingly and/or intentionally exposing any individuals to a chemical known to the state to cause cancer or reproductive toxicity.” These chemicals are listed in the Proposition 65 chemical list. Once a chemical becomes listed, businesses have twelve months to comply with the specified warning requirements. The warning can be applied in several ways, including:

  • Labeling the consumer products

  • Posting signs at the workplace

  • Distributing notices

  • Publishing notifications within newspapers

Both TCE and PCE are chemicals known by the State of California to cause cancer, birth defects or other reproductive harm, and appear on the Proposition 65 chemical list.

Safe Harbor levels have been established for many of the chemicals listed under Proposition 65. They include No Significant Risk Levels (NSRLs) for cancer-causing chemicals and Maximum Allowable Dose Levels (MADLs) for chemicals that cause reproductive toxicity. If exposure to a chemical occurs at or below the Safe Harbor level, businesses are exempt from the Proposition 65 warning requirements.

By using the Safe Harbor levels for TCE[5] and PCE[6], we can calculate the allowable indoor air concentrations of these chemicals for adults in a commercial/industrial setting. The calculation assumes an inhalation rate of 10 cubic meters per day (m3/day) in a workplace or 20 m3/day for a residential adult. This inhalation rate may be applied to the NSRL inhalation daily dose to determine an acceptable indoor air concentration in either a residential or commercial/industrial setting. For example, for PCE the NSRL is 14 micrograms per day (ug/day) and by dividing that level by the workplace inhalation rate the indoor air concentration would be 1.4 ug/m3. The NSRL is based on an acceptable risk level of one in one hundred thousand (1E-5) or a hazard quotient of 1.0.

NSRL ÷ Inhalation Rate = Conc Indoor Air

The Safe Harbor indoor air calculations for workplace scenario for TCE and PCE are 50 ug/day (inhalation) and 14 ug/day, respectively. The results of these are comparable to the TCE and PCE Screening levels and PCE Action Levels reported above - they are similarly low. If we calculate the estimated soil vapor concentrations using the EPA’s 0.03 attenuation factor, we find that the following soil vapor concentrations may trigger Proposition 65 warnings.

TCE: 5.0 ÷ 0.03 = 166.7 ug/m3

PCE: 1.4 ÷ 0.03 = 46.7 ug/m3

Obviously, you may want to directly measure indoor air concentrations prior to providing Proposition 65 warnings.


Lessons Learned

The main lesson to be learned from this exercise is that low soil vapor concentrations of TCE and PCE may trigger clean up requirements and Proposition 65 warnings due to vapor intrusion. In addition, the DTSC is requiring expedited mitigation actions for similar levels of TCE in indoor air. As you manage TCE and PCE levels in soil, soil vapor, and indoor air, be sure to not only keep the EPA and DTSC’s screening levels in mind but also consider Proposition 65 Safe Harbor levels for these chemicals and TCE Action Levels.

[1] USEPA Regional Screening Levels (RSLs) [2] April 2019 HERO Note 3 [3] OEHHA Fact Sheet: Trichloroethylene in Indoor Air [4] DTSC reminds us that possible ways to quickly reduce TCE levels in indoor air include increasing the ventilation or using an air purifier with an activated charcoal filter. (more on this in an upcoming client letter) [5] Prop 65 OEHHA information on Trichloroethylene [6] Prop 65 OEHHA information on Tetrachloroethylene (Perchloroethylene)


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