The dangers of manure gas and strategies for mitigation

Many farms are emptying out manure storages this fall and the potential for manure gas intoxication is ever present.

Tractor pumping out of a manure lagoon.
It is important to be alert and aware of your surroundings when working around manure storages as manure gases can become quickly overpowering.

Fall is quickly approaching and harvest season is almost upon us. With that in mind, many farms will be spreading manure and pits will be agitated and emptied. Because of the still warm temperatures and high humidity, bacterial activity increases in manure, directly increasing manure gas. Manure is an excellent and readily available source of fertilizer for many farms, however, it is important to consider the danger of gas that accompany working with manure.  In June of 2015, a father and son duo from Cylinder, Iowa were both killed from manure pit gas on their Iowa hog facility (Rodgers and Eller, 2015). During a routine pumping of manure from one of the hog facility pits, the son climbed down into the pit after dropping a piece of equipment and was immediately overcome by the manure gas. His father went in after him and experienced the manure gas as well. Unfortunately, neither survived. Similarly, in 2016, a Wisconsin farmer was agitating manure in an outdoor lagoon before spreading on fields and was also overcome by manure gas (Veselka, 2016).

These stories are not new news and serve to remind all of us about the importance of knowing what manure gas we need to be aware of and how we should respond in emergency situations.

What are the gases of concern and why are they dangerous?

Four gases of major importance are ammonia (NH3), carbon dioxide (CO2), methane (CH4) and hydrogen sulfide (H2S). These gases are produced by microbial activity within the manure from the microbial respiration that occurs (rather than use oxygen for respiration, bacteria utilize inorganic sources like nitrogen and sulfur). The table below shows some of the characteristics of each of these gases with paragraphs detailing each one as well.

Table 1: Characteristics and risk amounts of the manure gases: ammonia, carbon 
dioxide, methane and hydrogen sulfide

Gas Characteristics

Ammonia (NH3)

Carbon Dioxide (CO2­)

Methane (CH4)

Hydrogen Sulfide (H2S)

Color and Odor




Colorless/rotten egg smell

Heavier or Lighter than Air





OSHA* Long-term

(8-hour work period) Exposure Max (ppm)





OSHA Short-term Exposure Max (ppm)




15 (evacuation recommended)

Death Exposure (ppm)



50,000 (asphyxiation)


(Adapted from Donham et al., 2010. and National Ag Safety Database, n.d.)
*Occupational Safety and Health Administration 

Ammonia: sharp, pungent smell

Ammonia (NH3) gas in high concentrations can cause eye ulcerations and severe respiratory aggravation. While NH3 is typically not deadly, it is important to consider long-term exposure effects on respiratory health on those that are in proximity with it on a day-to-day basis.

Just as humans can suffer respiratory effects from inhalation of NH3, other livestock are susceptible as well. In swine, at only 50 ppm, there is an expected decrease in performance and health. Additionally, long-term exposure at 300 ppm+ will cause convulsions (Donham et al., 2010).

Carbon Dioxide: displaces oxygen

Carbon dioxide (CO2) may not appear to pose a threat like some of the other manure gases, however, it is dangerous from the perspective that it can replace the oxygen in your blood. Moderate concentrations of CO2 can lead to shortness of breath and dizziness (National Ag Safety Database, n.d.).

As this is a by-product of livestock respiration, animals in confined spaces can also be affected by asphyxiation from CO2 similar to people. That being said, when examining an eXtension article by Donham et al. (2010), it is important to note that humans can tolerate up to 260,000 ppm+ before death, while swine can only tolerate up to 200,000 ppm.  

Methane: highly flammable

Methane (CH4) is not a concern from a human respiratory standpoint.  If a building with manure storage is not ventilated properly, it can cause headaches and asphyxiation. Additionally, CH4 tends to build up in the foam that accumulates on the top of liquid manure and is highly flammable according to the Farm and Ranch eXtension in Safety and Health (FReSH) Community of Practice (2012). The explosive potential of CH4 is dangerous to both people and livestock within proximity of this gas.

Hydrogen sulfide: acutely dangerous

Hydrogen sulfide (H2S) is the gas most often associated with manure related deaths on farms and is considered to be the most acutely dangerous (National Ag Safety Database, n.d.). This gas travels readily along the ground and in confined spaces, like manure storages. It causes paralysis of the nerve cells in the nose, which deadens the smell at only 100-150 ppm (United States Department of Labor, n.d.). At 700-1,000 ppm, there is rapid loss of consciousness and death can occur in minutes. Additionally, even if someone is exposed to high concentrations of H2S for only a short amount of time, the reaction to the gas can be delayed up to 24 hours and can include pulmonary edema (fluid build-up in the lungs) possibly leading to death. Similarly, other long-term neurological effects from H2S exposure are possible.

Like its counterpart gases, NH3 and CO2, H2S is also a danger to livestock, specifically swine, in that it only takes about 20 ppm to start seeing signs of nervousness, fear of light and loss of appetite (Donham et al., 2010). When concentrations reach 200 ppm, swine may experience pulmonary edema and death shortly thereafter.

What are some of the signs of being overcome by manure gases?

While several signs of being overcome by manure gasses have been mentioned, there are others to be on the lookout for as well. Some of these signs include feeling hot and clammy, loss of motor skills, irregular/fast heartbeat, tightness of chest, panting, nausea/vomiting and anxiety (Meinen, 2016).

How can I measure manure gases?

There are several different types of manure gas monitors that can be utilized on the farm. The monitor used depends on the farm as well as the location of the manure storage and whether it is a confined or unconfined space. It is important to consider the type of gas you may come into contact with as well as the price that works in your budget. In table 2 below, a list of the different types of manure gas monitors are depicted by Farm and Ranch eXtension in Safety and Health (FReSH) Community of Practice (2012a) and the costs to purchase this equipment from the year 2011 (Steel et al., 2011).

Table 2: Manure gas monitor types, pros and cons, and prices

Monitor Type:




Electronic Installed

  • Accurate
  • Advanced alarm system
  • Not practical for confined storages (corrosion of monitor occurs more rapidly)

$2,000 to $4,000

Electronic Portable: Single-Gas

  • Relatively inexpensive
  • Less wear and tear compared to a fixed monitor
  • Only detects one gas

$250 to $700

Electronic Portable: Multi-Gas

  • Can measure several gasses at once including oxygen
  • Less wear and tear compared to a fixed monitor
  • More expensive than a single-gas monitor

$400 to $1,400

Gas Detector Tubes and Pump

  • Relatively inexpensive
  • Easy to use
  • Some tubes may only be used once
  • Typically only reads one gas (does not read oxygen), but can have cross-contamination from other gases

Pump: about $150

Tubes: $68/10-pack to $82/10-pack

Gas Detection Badge

  • Relatively inexpensive
  • Worn on body of worker, so portable and easy to carry
  • Only detects one gas
  • Can only be used one time

$141/box of 10 to $195/box of 50

(Adapted from Farm and Ranch eXtension in Safety and Health (FReSH) Community of Practice, 2012a and Steel et al., 2011)

What are ways to prevent a dangerous situation?

Follow manufacturer recommendations of equipment when agitating and handling manure in an enclosed pit to ensure:

  • Proper ventilation.
  • Fans are on and working.
  • Equipment is operating correctly.

When working around or near a manure pit:

  • Let someone know where you are and what time you are going. This allows a person to know right where to look if you are not back in a timely manner.
  • If someone you know or even an animal/pet is overcome by manure gas, do not go in after them unless you have proper respiratory protection.
  • Should you encounter a situation where someone goes down and is unconscious, immediately call 911 as first responders have the proper respiratory equipment and training to enter into these dangerous situations.
  • If it is available, wear a gas monitor or have one in the manure storage to detect manure gas concentrations that may be approaching dangerous, life-threatening levels.
  • When manure is being agitated, be aware of your positioning to the pit and where the manure gases are likely to settle.
  • It is also important to be cognizant of manure tankers and how easily manure gases can settle inside this type of small and confined space.
  • Gases have a tendency to settle inside tankers as well as leak out the top, which can pose a threat to those who examine and clean the tankers.
  • Wear personal protective equipment, like a proper fitting respiratory mask, if you go into a confined manure storage.

By understanding the dangerous gases found in manure, knowing the warning signs of a person who is experiencing high concentrations of manure gases and implementing safe practices when working around manure, there is the potential for fewer accidents and deaths. Who knows, you just may save a life, maybe even your life. 

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