Health Impacts Estimates

Aliso Canyon Gas Leak –
Preliminary Evaluation for Potential Health Impacts
[updated April 26, 2016]


The measurements conducted to date by SCAQMD and described in detail in the Ambient Air Analysis web page allow for preliminary estimates of the potential air toxic risks associated with the air pollutants emitted by gas leak at well SS-25 at the Aliso Canyon Natural Gas Storage Facility operated by SoCal Gas.  As described below, the levels of community exposure to air contaminants in natural gas that is released from the well SS-25 gas leak are not expected to cause a significant increase in overall risk of health effects from either short-term or long-term exposure and are similar to the levels of air toxic pollutants typically found in outdoor air in Southern California.  However, exposure to the odorizing compounds which are added to natural gas, such as tetrahydrothiophene and mercaptan, can cause short-term symptoms, consistent with many of the symptoms reported by some community members.  There are insufficient studies available to determine the long-term effects of exposure to these odorizing additives.

The estimates presented here are based on the limited amount of data that are currently available, and, as such, these results should be considered preliminary.  For instance, long-term health risk estimates based on measurements typically use at least one full year of monitoring data, while in this case, results from only a couple months of time-integrated samples are available.  SCAQMD will continue to assess the monitoring data as additional results are available, and these estimates will be updated if substantially higher concentrations are reported. SCAQMD has also evaluated and considered the results of other ambient air monitoring efforts conducted by CARB, SoCal Gas and Los Angeles Unified School District, as noted in the Other Monitoring Efforts section below.

Methods for monitoring and assessment of health risks

The preliminary results presented here are based solely on the monitoring data collected by SCAQMD at select locations in the communities near the leaking well.  The state Office of Environmental Health Hazard Assessment (OEHHA) also has conducted an analysis of monitoring data collected by others, including SoCal Gas. 

SCAQMD has employed multiple sampling methods to collect data related to the gas leak incident.  Some samples are collected “instantaneously” over a short period of time, while other samples are collected over longer periods of time (i.e. 24 hours) on a set schedule (“integrated samples”). Instantaneous samples were collected in many different locations within the community since late October 2015. The 24-hour integrated sampling for air toxics began at one location (shown as “Site #3” on the map) on December 21, 2015. A second site (shown as “Site #6” on the map) began sampling for air toxics on January 16, 2016, and a third site (shown as “Site #4” on the map) was added on February 24, 2016.  These monitoring results have been used in two ways to evaluate potential short-term (“acute”) and long-term (“chronic”) health effects.  First, health risks were calculated based on the pollutant concentrations at the monitoring locations in the community.  Second, health risks were calculated by using the monitoring results taken from inside the Aliso Canyon facility approximately 10 feet from the SS-25 well, in addition to the California Air Resources Board’s estimates of the total methane leak rates, entering this information into a computer model, and estimating what the pollutant concentrations and resulting health risks are throughout the community.  This modeling analysis extended out approximately four miles into the community, though the areas with the highest expected impacts are found closest to the facility.  It is important to note that there are uncertainties with these modeling results given the very limited number of samples available to estimate toxic emissions directly from the well.   However, the preliminary modeling results still provide an initial estimate of potential health risk.

Health risks are typically presented in terms of non-cancer risks and cancer risks.  For non-cancer risks, estimated exposures can be compared to reference exposure levels (RELs) established by OEHHA.  An REL is established at the concentration of the pollutant at or below which adverse health effects are not expected to occur, including for those people who are more sensitive to these effects than the general population.  RELs are established for acute and chronic exposures separately.  In addition, estimates of cancer risks can be made based on established OEHHA methods, and interpreted as the chance that being exposed to small amounts of these chemicals over a specified time period will cause cancer.  For the current analysis of health risks from the gas leak, “chronic exposures” are assumed to be six months long, because OEHHA guidelines recommend that exposures lasting between 2 and 6 months be calculated assuming a six month exposure duration. Given the announcement by SoCal Gas on February 11, 2016 that the Company had temporarily controlled the flow of gas from the leaking well, and the announcement on February 18, 2016 by the California Division of Oil, Gas and Geothermal Resources (DOGGR) that the well had been permanently sealed, the actual exposure duration to air toxics while the well was actively leaking is likely less than six months.

As shown in the Ambient Air monitoring results conducted by SCAQMD, higher levels of certain air toxics were found in some instantaneous samples in the Porter Ranch community, and the same air toxics were identified in a sample taken about 10 feet from the SS-25 well.  The identified air toxics were included in these risk estimates, but the results indicate that the vast majority of any potential acute or chronic toxic health risks are driven by exposures to benzene.  These preliminary risk estimates do not include effects from exposure to tetrahydrothiophene and mercaptans, because there are no established RELs or cancer toxicity values available for these compounds given the lack of sufficient published scientific information.  Nonetheless, the SCAQMD and other public health agencies acknowledge, based on the limited information available, that exposure to the odors from these additives can cause short-term physiological responses consistent with many of the symptoms reported by some community members.

Short-term (acute) health risk

To assess short-term, or acute, risk, results of the instantaneous samples from the community locations were used.  One particular sample contained the highest levels of benzene and other air toxics out of more than 70 instantaneous community samples collected to date by SCAQMD.  Using the concentrations from this sample, collected on October 26, 2015 on Kilfinan Street in Porter Ranch, between Turtle Ridge Way and High Glen Way, the calculated acute health risk is approximately one-third of the REL.  In other words, the highest short-term air toxic levels measured by SCAQMD are well below the levels where adverse health effects are expected to occur.  It is important to note that, because instantaneous samples are taken at discrete locations over a very short time period, the samples may not have captured the highest levels of air toxic exposure in the community.  SoCal Gas reported measuring the highest benzene levels (5.55 ppb) on November 10, 2015, out of the over 1,000 instantaneous grab samples they collected through February 13, 2016.  This higher benzene level measured by SoCal Gas results in a higher estimate of acute health risk, but the risk would be approximately two-thirds of the acute REL, still below the levels where adverse health effects are expected to occur.

SoCal Gas also measured hydrogen sulfide in instantaneous samples in the Porter Ranch community. The majority of these samples had concentrations that were too low to be measured, but 6 community samples had detectable levels of hydrogen sulfide. Among these 6 samples with detectable concentrations of hydrogen sulfide, five samples had concentrations well below the acute REL of 30 ppb, and only one sample (collected on November 12, 2015) exceeded the acute REL. This air sample contained a hydrogen sulfide concentration of 183 ppb.  This high level has not been detected in other samples. Potential effects (e.g. headaches, nausea, respiratory irritation) of such a short-term exposure are expected to be short-lived.

One sample collected about 10 feet from the well SS-25, along with California Air Resources Board’s estimates of the total methane leak rates, provides a rough estimate of the emission rate of toxics from the well.  This estimate was used in an air quality computer model to estimate exposures throughout the surrounding communities within about four miles of the leak.  These preliminary modeling results also show acute health risk well below the REL throughout the community, so adverse acute health effects from these air toxics are not expected. 

Long-term (chronic) health and cancer risk

To assess potential long-term non-cancer and cancer risk, data from all the available 24-hour integrated samples collected by SCAQMD were averaged, separately, for Sites #3, #4, and #6. Data from the SCAQMD air samples can be found here.  The estimates presented here are based on an assumed six month constant exposure.  Based on these limited data, the levels of benzene and other air toxics found at this location in the community are within the range typically seen throughout the Southern California region.  However, it is important to note that long-term risk estimates based on measurements typically use at least one full year of data, while in this analysis, only a limited number of samples were available, though sample collection efforts are ongoing.  The monitoring data will be assessed as additional results are available, and these estimates will be updated if substantially higher concentrations are reported.

Health risks were estimated based on the amounts of air toxics measured in the community, and can be interpreted as the health impacts of these air toxics due to the gas leak as well as typical urban sources of the same pollutants, such as emissions from cars and trucks.  Based on SCAQMD 24-hour integrated samples, the estimated long-term non-cancer health risk was less than one-third of the REL, well below the levels where adverse health effects are expected to occur. Additionally, using the same SCAQMD measurement data in the community, the estimated cancer risk is roughly between 2-in-one million and 5-in-one million due to benzene from all sources, including the leaking well and other sources such as cars and trucks, if exposures persisted for six months.  Based on previous SCAQMD studies, the estimated average residential cancer risk in the Porter Ranch area from exposure to all ambient air toxics not related to the gas leak over 30 years is in the range of 400-in-one million to 500-in-one million.

SoCal Gas began collecting 12-hour samples and analyzing for hydrogen sulfide in January 2016, and all the samples reported thus far have been below detection limits. SCAQMD has monitored for hydrogen sulfide beginning in January 2016; results from this monitoring showed levels that were below the chronic REL of 8 ppb.

To estimate the long-term risk attributable to emissions from the gas leak alone, the same air quality model described previously was used.  Modeling results indicate that the chronic non-cancer risks from the gas leak are well below the chronic REL.  The modeled additional cancer risks attributed to the gas leak are estimated to be less than 2-in-one million for a six-month exposure, almost exclusively due to benzene. 

Other Monitoring Efforts

On January 11, 2016, SoCal Gas began collecting 12-hour integrated air samples at several locations within the boundaries of the Aliso Canyon facility property, including locations along the fenceline.  The SoCal Gas sampling results can be found here.  The analysis included only the samples from the fenceline locations because these are more representative of community exposures.  Compared to the community-based sampling locations used by SCAQMD, these SoCal Gas sampling locations along the fenceline are closer to well SS-25, and therefore are likely to be more impacted by emissions from the gas leak.  On March 3, 2016, SoCal Gas began collecting 24-hour integrated air samples instead of the 12-hour samples. Based on the limited available SoCal Gas monitoring data, preliminary estimates of the long-term health risks from a six-month exposure to benzene and air toxics from all sources (including the gas leak) is less than one-third of the REL, and the estimated cancer risk from benzene is less than 4-in-one million.

The California Air Resources Board (CARB) began reporting hourly benzene data on February 1, 2016. Information on the monitoring conducted by CARB can be found here. The estimated health risks based on these benzene data from CARB are similar to the risks estimated using other available data as reported above.

The Los Angeles Unified School District (LAUSD) also collected air toxics samples at schools in the nearby community. Information on the sampling conducted by LAUSD can be found here.  The 8-hour samples appear to be samples of indoor air at the school facility, and do not represent community exposures.  Therefore, these data should not be used to estimate health risks in the community.

Next steps

The SCAQMD’s independent Hearing Board approved a legal order that requires SoCal Gas to fund a health study of the potential health effects of exposure to the gas leak.  This includes exposure to the odorants added to natural gas, for which there are currently no established RELs or cancer toxicity values.

Finally, monitoring and analysis will continue, and the above risk information will be updated if conditions change, or if substantially higher air pollution concentrations are reported.