It only takes a few moments to be overcome by hydrogen sulfide (H₂S) gas. Paralysis of the respiratory system occurs almost instantly, without recognition that there is even a problem. Many people recognize hydrogen sulfide by its rotten-egg odor, but the odor is only detectable at low levels that can be irritating and offer motivation for escape. Of greater concern are the toxic gas levels present in and around manure storages that can knock a person to the ground in respiratory distress. Hydrogen sulfide acts as a chemical asphyxiant on oxygen utilization and the central nervous system.

Gas monitoring is always recommended when working around toxic gases such as hydrogen sulfide. Fortunately, affordable portable personal gas monitors are available for on-farm use. Of course, there is a hierarchy of preventive measures one can use when working around toxic atmospheres. Layers of control are recommended, such as ventilation of the area, hazard identification, and personal protection equipment (PPE: masks, respirators). The personal gas monitor identifies potentially hazardous environments and warns the user when to escape. This fact sheet will inform you about some of the important details to consider when obtaining a personal gas monitor for hydrogen sulfide.

Figure A: Dangers of hydrogen sulfide gas are well known outside agricultural venues, such as in the gas industry and manufacturing, and partially account for manufacturers offering a wide selection of personal gas monitors. Source: Adapted from OSHA.

Figure B: Sign reminding operators of need for safe conditions and use of gas detectors during manure agitation events. Source: USDA NRCS Pennsylvania

Choosing a Personal Gas Monitor

Portable gas monitors alert the user to a hazardous situation with a combination of audible noise, visual LED light, and vibration alarm signals. Even in noisy or bright environments the user can be warned of dangerous H₂S levels by feeling the vibration. Alarms can often be set by the user to alert them of dangerous gas concentrations that are not otherwise detectable by smell or sight. Low and high alarm levels for H₂S are typically preset well below lethal concentrations at warning levels of 10 and 15 ppm, respectively. Table 1 lists hydrogen sulfide levels of concern and safe limits. Choose a personal H₂S gas monitor that includes a digital readout displaying the measured gas concentration, which is helpful when assessing risk when an alarm sounds. For routine manure agitating, pumping, hauling, and wash-down operations performed above ground, inside a barn with slotted flooring above a manure pit, or from a tractor cab, a single-gas H₂S monitor may be adequate. Single gas monitors are about the size of a cell phone and typically cost under $300. Helpful features include multi‐year battery life and display of gas concentration.

Table 1. Hydrogen sulfide limits for workday exposures from three safety agencies.

Source: Adapted from OSHA Hydrogen Sulfide Standards
ACGIH: American Conference of Governmental Industrial Hygienists
NIOSH: National Institute for Occupational Safety and Health
OSHA: Occupational Safety and Health Administration

Figure C: A single-gas personal monitor is a small, wearable, and affordable safety device available for hydrogen sulfide (and other gases) from several manufacturers. Source: E. Fabian

For professional manure haulers, a multi-gas monitor can be equipped with sensors that measure different hazardous gases such as methane, low oxygen level in a confined space, and carbon monoxide (exhaust) from equipment operation, in addition to hydrogen sulfide readings. Sometimes methane is expressed in terms of LEL, or lower explosive limit of combustible gases. The cost of multi-gas monitors is about double or triple that of single gas units.

Figure D: Examples of single-gas and multi-gas monitors that are useful on the farm. Source: E. Fabian

Since confined spaces such as manure pits often contain oxygen-deficient atmospheres, multi-gas meters should be used to check and monitor conditions before and during confined space entry (for more information, see the Penn State Extension fact sheet “ Confined Space Manure Gas Monitoring ”.) Consider meters that can be equipped with a slim flexible tube and sampling pump that can be inserted into a suspicious area to test for toxic conditions before human entry.

Figure E: Meter outfitted with sampling tube attached to internal pump in a multi-gas meter used to determine conditions before considering entry into a remote location, such as a sump or pit. Note weight taped on the end to assist in dropping the sample tube. Source: E. Fabian

Most personal single-gas H₂S monitors are dustproof and waterproof (to a depth of 1 meter) and are intrinsically safe, which means that the monitors won’t spark or become hot enough to cause an explosion or fire in the presence of combustible or flammable gases or liquids. Many personal H₂S gas monitors will store 25 to 60 of the last alarm events in memory. Certain models include dataloggers that can record a month or more of continuous measurement data. Retrieving logged data usually requires a docking station or infrared adapter. Most companies should be able to send you the data files after you return a rental unit. Data may be valuable for tracking worker exposure and to implement practices that improve safety.

Using a Gas Monitor

Personal H₂S gas monitors are equipped with a clip or ring to attach the monitor to clothing. It is recommended that personal gas monitors be worn in the breathing zone, so the monitor is attached to a collar, lapel, or chest pocket with the gas sensor uncovered and unobstructed.

The most important action to take when an alarm goes off is to leave the area! You can always stop and inspect the digital readout to assess the hazard from a safe distance in fresh air. It may take a short duration of time for the alarm to turn off and for the measured concentration to return to zero. Always be sure to have an escape path from the toxic area. Retreating to a safe area may be as simple as walking outside of a confined space, or it may require walking 50 feet or more away from a manure storage undergoing agitation prior to pumping.

Figure F: Dairy manure tank agitation showing warning sign and operator use of a single-gas hydrogen sulfide personal gas monitor. Source: E. Fabian

Figure G: Setting up manure agitation equipment while wearing single gas monitor for hydrogen sulfide detection. Source: E. Fabian

Purchase or Rent a Gas Monitor

Upkeep comes with ownership of a personal H₂S gas monitor. Monitors should be stored in a room or container that is free from contaminant gas since continual exposure to even low levels of H₂S and other contaminant gases will degrade the gas sensor more rapidly. Some monitors have optional “hibernation cases” that prolong sensor life when they will not be used for an extended period of time (weeks or months).

Personal gas monitors must be periodically “bump tested” to verify the sensors are detecting the correct gas concentrations and ensure the monitor will alarm at the appropriate levels. A bump test is performed by exposing the sensor to a known concentration of H₂S gas, typically 25 ppm. To perform the bump test, a bottle of calibration gas (commonly referred to as “cal gas”) is needed. A length of tubing or special clip is attached to the gas monitor to expose the sensor to the gas. The gas monitor passes the test if the alarms are triggered within a specified duration of time, typically less than one minute. The cost of cal gas bottles varies widely depending on supplier and gas mixture and concentrations, but figure at least $100. Cal gas is necessary for upkeep of the monitor and safe use but adds to the annual cost of meter ownership since cal gas bottles have expiration dates that need to be valid.

Figure H: Docking station for “bump testing” gas meter to ensure reliable monitoring and for data downloading and charging options. Some manufacturers offer meter rental that includes maintenance and calibration, which eliminates the need for an on-farm docking station and calibration gases. Source: E. Fabian

If you do not need to use a personal H₂S gas monitor on a regular basis, consider that many companies offer rental units. The monitor is maintained by the manufacturer or supply company, and calibrated before being shipped to you for use. Rental costs may be as low as $15 weekly or $45 monthly for individual single-gas H₂S monitors (Industrial Scientific Tango TX1 includes prepaid return shipping). Some companies also rent accessories such as docking stations and calibration equipment needed to perform regular bump tests and occasional recalibration.

Additional Information

Hydrogen sulfide is toxic at extremely low concentrations. It smells like “rotten eggs” at low concentrations but quickly overcomes our sense of smell (Table 2). It is flammable and colorless, but its most challenging trait is its density. Hydrogen sulfide is heavier than air and tends to accumulate in low-lying areas, including manure sumps, deep pits, and even shallow depressions around an outdoor manure storage. Pockets of the gas can occur anywhere or carried by wind.

It is highly recommended that farm operators working around manure storages with gypsum (calcium sulfate) bed-ding wear a hydrogen sulfide personal gas monitor due to high H₂S release during manure agitation. Refer to fact sheet “ Safety Risk from Manure Storages of Dairy Cows Bedded with Gypsum ” for an overview of hydrogen sulfide levels and potential risks associated with gypsum bedding use.

Table 3 provides a list of several manufacturers of personal gas monitors with contact information. Additional information for single and multiple personal gas monitor manufacturers, models, distributors, and prices is available at Gas Monitors For Consideration in Swine Barn Activities (High‐Hazard H2S and Methane Operations), Great Plains Center for Agricultural Health.

Table 2. Symptoms and responses to exposures of various levels of hydrogen sulfide gas.

Source: Hydrogen Sulfide Hazards, OSHA

Table 3. List of personal gas monitor manufacturers.

Prepared by Dan Hofstetter, extension research assistant; Eileen Fabian, professor of agricultural engineering; and Michael Pate, Nationwide Insurance Associate Professor of Agricultural Safety and Health.