The primary component of natural gas is methane, a highly combustible, yet completely odorless gas. Natural gas is intentionally odorized using a variety of sulfur-based odorant compounds to aid in leak detection. Without odorants, gas leaks would be completely undetectable.
To date, no comprehensive assessments have been performed on toxicity or human health risks of commonly used natural gas odorants. In this rigorous literature review, published in Current Environmental Health Reports, researchers evaluate the scientific evidence on the toxicological and human health outcomes of the most widely used natural gas odorants. A total of 22 articles were identified, of which 10 were human exposure studies, 11 were animal studies, and one was a cell-culture study.
Due to the lack of transparency in chemical disclosures, a definitive understanding of natural gas odorant usage is limited to documented sulfur-based compounds and English-written peer-reviewed studies of associated health effects. Researchers also did not include gray literature, government reports, industry reports, or toxicity assessments beyond the exposure events from the loss of containment event at the Aliso Canyon Underground Gas Storage Facility in Porter Ranch, California and at the wastewater treatment facility in Fairburn, Georgia.
Odorants are critical for leak detection and intended to be inhaled for a very short time, however, they may pose health risks at exposures much lower than currently suspected.
For each of the five commonly used natural gas odorants investigated (except isopropyl mercaptan, due to lack of evidence), this review indicates:
- Sulfur-based odorant compounds can induce a range of adverse symptoms at very low concentrations.
- These symptoms range from headaches to respiratory inflammation to skin rashes.
- Very little data has been collected on exposure concentration, limiting certainty of the health effects and toxicity of odorants.
Researchers also found a surprising lack of established regulatory health and exposure benchmarks given the widespread use of the odorants studied. Overall, evidence exists for both short-term self-reported health symptoms associated with odorant exposure and longer-term exposure associated with clinically diagnosed chronic obstructive pulmonary disease (COPD).
To improve understanding and management of natural gas odorants with respect to human health, researchers recommend the following:
- Require chemical disclosure of odorants used in natural gas, with corresponding health-based recommended exposure limits.
- Studies are needed to better understand causes of symptoms associated with odorant exposure, including effects on potentially susceptible populations or those that may exhibit some forms of odorant sensitivity or insensitivity.
- Risk assessments should be undertaken to develop general exposure limits for commonly used odorants beyond the use of occupational exposure limits.
- To reduce natural gas leaks and odorant exposures, real-time methane sensor detectors are needed similar to smoke detectors, particularly for high occupancy buildings, commercial kitchens, gas utility work sites, and any other setting involving sensitive subgroups.