What is Ethanethiol* and Why Does It Matter?

Ethanethiol (also called ethyl mercaptan) is the chemical added to natural gas and propane to give them that distinctive rotten-egg or skunk-like smell. Without it, gas leaks would be odorless and far more dangerous. It's a sulfur-based compound that works at incredibly small concentrations — your nose can detect it at levels below 1 part per billion. In drinking water, ethanethiol can enter through industrial runoff, leaks near gas processing facilities, or contamination from petroleum-related operations. It's not a naturally occurring contaminant. Its presence in tap water is almost always tied to industrial activity nearby.

Current data shows ethanethiol has been detected in 31 water systems, with an average concentration of 2.647 parts per billion (ppb) and a maximum detected level of 3.4 ppb. The good news is that none of those 31 systems tested above any established health guideline. That means the detected levels fall within ranges considered low-risk by current standards. Still, detection matters. Finding any industrial chemical in drinking water is worth understanding, especially for households with young children, pregnant women, or anyone with chemical sensitivities.

At very low levels, ethanethiol is not considered acutely toxic to humans. Most research on its health effects comes from inhalation studies rather than ingestion studies, which means data on drinking water exposure is limited. At higher concentrations — well above what's currently detected in these systems — it has been associated with headaches, nausea, and irritation of the mucous membranes. Long-term low-level exposure is less studied, which is part of why monitoring still matters even when levels appear safe. The EPA has not established a Maximum Contaminant Level (MCL) specifically for ethanethiol in drinking water, and the EWG has not published a separate health guideline for it either. Its presence in water systems is tracked, but regulatory attention remains limited compared to contaminants like lead or chromium-6.

Geographically, all 31 systems where ethanethiol has been detected are located in Texas. That pattern makes sense. Texas is home to some of the largest natural gas processing, petrochemical refining, and oil production infrastructure in the country. Facilities that handle or process natural gas use ethanethiol as an odorant additive, and industrial sites near water sources can contribute trace amounts to local water supplies through runoff or accidental discharge. Communities near the Gulf Coast industrial corridor or the Permian Basin are particularly worth watching. If you live in Texas and rely on municipal water, checking your annual water quality report — also called a Consumer Confidence Report — is a smart first step.

When it comes to removing ethanethiol from drinking water, activated carbon filtration is one of the most effective options available. Carbon filters work by adsorption, meaning the contaminant molecules stick to the surface of the carbon as water passes through. For a more thorough approach, a reverse osmosis (RO) system removes a broad range of organic compounds, including sulfur-based chemicals like ethanethiol. Reverse osmosis pushes water through a semi-permeable membrane, filtering out contaminants at the molecular level. Echo Water's reverse osmosis systems are designed to handle exactly this kind of low-level industrial contamination, giving you cleaner water at the tap without relying on bottled water. If you're in Texas and your water tests positive for ethanethiol — even at levels below guidelines — a point-of-use filter under your kitchen sink is a practical, affordable solution worth considering.

Regulatory Standards for Ethanethiol*

Cities With the Highest Ethanethiol* Levels

States Most Affected by Ethanethiol*

Frequently Asked Questions

Related Contaminant Guides