In today’s construction and environmental landscape, assessing the safety of buildings goes beyond structural integrity. One of the most invisible but potentially dangerous threats is the presence of hazardous gases. Whether emitted from natural sources like soil or from human activities, these gases can pose serious risks to both the occupants’ health and the building’s structure. At Avada Environmental Ltd., we understand the importance of identifying, assessing, and mitigating these risks to protect people and property.
This guide offers a deep dive into the process of evaluating the dangers posed by hazardous gases in buildings. We will explore various testing methods, discuss remediation strategies, and focus on the Wilson Card Methodology—a critical framework for gas risk assessment. Whether you’re a developer, property manager, or homeowner, this narrative will walk you through how we identify risks and provide necessary solutions.
Understanding Hazardous Gases in Buildings
While gases are invisible to the naked eye, their impact can be substantial. Hazardous gases such as carbon dioxide, carbon monoxide, methane, hydrogen sulfide, and radon are often colorless and odorless, making them difficult to detect without specialised equipment. These gases may enter buildings from natural geological processes like soil and rock decay or from human activities such as landfill operations or industrial emissions. Each gas behaves differently, posing unique risks. For instance, radon, which comes from the natural decay of uranium in the ground, is known to increase the risk of lung cancer, while methane, produced from organic decomposition, can accumulate and become an explosion hazard if it builds up in confined spaces.
The sources of these gases vary widely. Radon seeps into buildings from the soil, particularly in areas with high uranium content in the ground. Methane, a by-product of decomposing organic material, is a common concern in areas near landfills or marshlands. Even common construction sites may hide hazards, as decomposing coal seams can release both methane and carbon dioxide. Meanwhile, gases like carbon monoxide are primarily the result of combustion activities, and hydrogen sulfide is often produced from the breakdown of organic matter in environments like sewage systems. All these gases pose potential health risks, including respiratory issues, neurological damage, or even death in extreme cases. Some can also affect the building structure, leading to corrosion or weakening over time.
The Process of Risk Assessment
Before testing begins, it’s crucial to conduct a detailed site evaluation to understand the potential sources and pathways through which hazardous gases might enter a building. The assessment starts with historical research of the site to identify past activities that could contribute to gas emissions. For example, if the building sits on land that was once a landfill or industrial site, there may be lingering gases underground. A geological survey is also essential to understand the site’s soil and rock composition, as some soil types are more permeable, allowing gases to move through them more easily.
Identifying potential pathways is a key part of the assessment. Gases can enter buildings through cracks in the foundation, openings around utility conduits, or permeable materials in walls and floors. Even in well-constructed buildings, pressure differences between indoor and outdoor environments can create a “suction” effect that draws gases into the structure. This migration of gases is influenced by soil properties, environmental conditions, and even changes in atmospheric pressure. As such, every assessment must consider how gases might move from their source into occupied spaces.
Once the initial site conditions are understood, the next step is to assess the risks by measuring gas concentrations and analyzing how they might impact the building and its occupants. This involves detailed sampling and the use of sophisticated monitoring equipment. However, the mere detection of gas isn’t enough; the data must be contextualised. Understanding the concentrations of gases and their potential health effects is vital for determining the extent of the threat.
Testing Methods
There are various methods for testing the presence of hazardous gases in buildings, and choosing the appropriate technique depends on the type of gas and its source. One common approach is active soil gas sampling, where probes are inserted into the ground at various depths to collect gas samples. These are then analysed to determine the presence of harmful gases like methane, carbon dioxide, and radon. This method helps assess the likelihood of gases migrating from the soil into the building structure.
Indoor air sampling is another method, particularly useful for detecting gases like carbon monoxide or VOCs (volatile organic compounds), which may accumulate inside enclosed spaces. Here, air samples are taken from within the building and analysed to determine gas concentrations. Portable detection devices or gas chromatographs are commonly used to ensure accuracy. These sampling strategies are often repeated over time to capture variations caused by environmental changes, such as shifts in temperature or atmospheric pressure.
Interpreting the data from these tests requires comparing gas levels to safety standards. For example, indoor carbon monoxide levels should not exceed 9 parts per million (ppm) over an 8-hour period according to the EPA. Elevated levels of certain gases, such as carbon dioxide above 5%, would immediately trigger concerns, as this could indicate both a health risk and the need for urgent remediation.
The Wilson Card Methodology
A key tool for gas risk assessment is the Wilson Card Methodology. Developed by Drs. Wilson and Card in the 1990s, this methodology provides a standardised framework for evaluating the risks posed by ground gases. The Wilson Card Methodology combines two critical parameters: the concentration of gases and their flow rates. By measuring both the amount of gas present and the speed at which it moves into a building, this method allows for an accurate risk assessment. The methodology categorises sites into “Characteristic Situations” (CS), ranging from CS1 (very low risk) to CS6 (very high risk).
For instance, consider a scenario where a building site is near a former landfill. Testing reveals methane concentrations of 5% by volume, along with carbon dioxide levels of 2%, and a gas flow rate of 10 liters per hour (l/h). Using the Wilson Card Methodology, we analyse the risk based on these figures. Methane at 5% approaches the lower explosive limit, while the flow rate of 10 l/h is moderate. In this case, the site might be classified as CS3, which represents a moderate risk. This classification informs the protective measures needed during construction, such as gas-resistant membranes or ventilation systems.
By following the Wilson Card Methodology, we can provide clear, actionable data on gas risks. For a site with even higher gas concentrations or faster flow rates, such as 7% methane and a flow rate of 25 l/h, the classification might rise to CS4 or higher, indicating a much more serious risk and the need for advanced mitigation strategies. This framework ensures that developers, builders, and property owners understand the risks they face and the level of protection required.
Risk Assessment in Detail
To fully understand the scope of hazardous gas risks, it’s important to consider both the concentration of specific gases and their flow rates. For example, carbon dioxide (CO₂) is naturally present in the environment at low concentrations, but elevated levels indoors can indicate poor ventilation or other issues. At concentrations above 1.5%, CO₂ can cause headaches, dizziness, and even unconsciousness. A gas monitoring system detecting CO₂ concentrations at 2% would immediately trigger alarm bells and suggest a significant ventilation issue or underground gas migration.
Similarly, carbon monoxide (CO) is particularly dangerous due to its ability to bind to hemoglobin in the blood, preventing oxygen transport. Even relatively low concentrations—above 9 ppm—can lead to poisoning if exposure is prolonged. Methane (CH₄) presents a unique risk because it is both flammable and explosive at concentrations between 5% and 15%. Even a 4% concentration would raise concerns, especially in poorly ventilated areas where accumulation could lead to dangerous conditions.
The flow rate of these gases is equally important in understanding the severity of the risk. For example, a methane concentration of 5% combined with a flow rate of 10 l/h suggests moderate gas migration into the building, but if that flow rate increases to 25 l/h, the potential for a hazardous buildup becomes much greater. Hydrogen sulfide (H₂S), known for its rotten egg smell, is toxic even at low concentrations (above 100 ppm can be immediately dangerous). Understanding the flow rate helps determine how quickly gases are accumulating and whether emergency remediation is necessary.
Remedial Options
Once the risks are assessed, it’s crucial to implement effective remediation strategies to protect both the building and its occupants. One of the most common approaches is ventilation, which can be either passive or active. Passive ventilation systems rely on natural airflow to disperse gases. For example, vent pipes or under-floor voids can be used to allow gases to escape before they enter the building. In contrast, active ventilation systems use mechanical fans or blowers to create negative pressure beneath the building, drawing gases away from occupied areas.
Gas-resistant membranes are another critical defense, particularly in new construction projects. These membranes are installed beneath the floor slab, forming a barrier that prevents gases like methane or radon from migrating into the building. Proper installation is crucial—any gaps or penetrations in the membrane could compromise its effectiveness, allowing gases to seep through.
For sites with high flow rates or particularly dangerous gas concentrations, sub-slab depressurization systems may be required. These systems reduce the pressure under the building, preventing gas migration into indoor spaces. In some cases, chemical oxidation methods are used to break down hazardous gases directly in the soil, but these are typically reserved for sites with extreme contamination.
Regardless of the remediation method chosen, ongoing monitoring is essential. Gas concentrations and flow rates can change over time due to environmental factors or changes in the building’s structure. Continuous monitoring systems can provide real-time data on gas levels, alerting occupants and building managers to any sudden changes. Regular inspections and maintenance of installed systems are also critical to ensure long-term safety.
Case Study: Applying the Wilson Card Methodology
To better understand how the Wilson Card Methodology is applied in real-world scenarios, consider a development project on a former coal storage yard. Initial site investigations revealed elevated levels of methane and carbon dioxide in the soil. Testing showed methane concentrations of 3%, carbon dioxide concentrations of 1%, and a flow rate of 15 l/h. Based on these figures, the site was classified as CS3, indicating a moderate risk.
As a result, the development team implemented a range of protective measures. A gas-resistant membrane was installed beneath the foundations to prevent gas ingress, and a passive ventilation system was designed to allow gases to safely escape from beneath the building. Gas detectors were also installed in key areas to provide ongoing monitoring.
Through these actions, the site was successfully remediated, and construction proceeded safely. The developers adhered to regulatory requirements while ensuring that future occupants would not face health risks due to hazardous gases. This case highlights the importance of thorough testing and the value of a structured methodology like the Wilson Card in managing gas risks.
Conclusion
Hazardous gases pose a unique and often overlooked threat to buildings and their occupants. Without proper assessment and remediation, these gases can lead to serious health risks and compromise building integrity. By using comprehensive risk assessment techniques and structured methodologies like the Wilson Card Method, it is possible to identify potential dangers early and implement effective measures to mitigate them.
At Avada Environmental Ltd., we specialise in assessing and addressing the risks posed by hazardous gases. Our team of experts uses cutting-edge technology and proven methods to ensure the safety of every site we evaluate. Whether you’re planning a new construction project or concerned about an existing building, we can help you navigate the complexities of gas risk assessment and remediation.
Protect your property and your people—reach out to us today to learn more about our services and how we can assist with your hazardous gas assessment needs.