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The PPM Blog

Vapor Intrusion: The Risk that Still Gets Underestimated

a man wearing a suit and tie smiling at the cameraContributed by Ben Lightsey P.E., Project Manager, PPM Consultants

Vapor intrusion is a commonly overlooked topic in environmental due diligence and redevelopment. It is not new, obscure, or technically mysterious. In fact, the pathway is well understood and often mimics another common environmental risk, radon. Chemical vapor intrusion, as discussed here, involves volatile chemicals in contaminated soil or groundwater moving as vapors through the vadose zone and entering buildings through cracks, utility penetrations, sumps, floor drains, elevator pits, and other preferential pathways. Yet despite guidance from multiple regulatory agencies and professional service groups, vapor intrusion is still too often discovered late: after a Phase I has been completed, after site plans are advanced, or after construction budgets are already under pressure. PPM provides related environmental due diligence and vapor intrusion assessment and remediation support for project teams evaluating these risks.

Vapor Does Not Respect Property Lines

One reason vapor intrusion is missed in early assessments is that the contamination source is not always on the subject property. A “clean-looking” parcel of land can be affected by a former dry cleaner, gasoline station, industrial user, or plume located in the area. Traditional due diligence has often focused on soil and groundwater impacts within property boundaries, but vapor does not respect those boundaries. ASTM E2600-22 was developed specifically to guide vapor encroachment screening for real estate transactions where chemicals of concern may migrate as vapors into the vadose zone of a property. The establishment of a separate screening framework is a reminder that conventional environmental site assessments may not fully address the possibility of vapor intrusion.

Why Indoor Air Data Can Be Misleading

Evaluating the pathway can also be complicated because indoor air data can be “noisy.” Background sources like cleaning products, paints, stored chemicals, fuel, vehicle exhaust, consumer products, and even occupant behavior can obscure whether detected compounds are coming from beneath the slab or from inside the building. On the flip side, a single “clean” indoor air sample can create false comfort, particularly where chlorinated solvents such as tetrachloroethene or trichloroethene are involved. Seasonal variability, building pressure, HVAC operation, groundwater elevation, barometric pressure, temperature differences, and precipitation can all change vapor movement, further complicating the picture.

Timing Matters in Redevelopment

Another common problem is timing. Early due diligence assessments are often conducted before the final site design is known. An early assessment may evaluate existing buildings, while the redevelopment plan calls for demolition, new foundations, underground parking, vapor-sensitive residential units, or a change from industrial to residential use. A condition that appears manageable under one building configuration can become significantly more difficult under another.

Regulatory Expectations Continue to Evolve

Regulatory expectations are moving in the opposite direction: toward more robust evaluation, more conservative screening, and more defensible documentation. EPA’s technical guide for assessing and mitigating the vapor intrusion pathway is intended for sites evaluated under CERCLA, and EPA also has made available brownfields vapor intrusion resources for redevelopment stakeholders. Many state programs have likewise become more sophisticated, often expecting multiple lines of evidence, deeper evaluation of preferential pathways, building-specific conceptual site models, and long-term stewardship for mitigation systems. The ITRC Vapor Intrusion Toolkit and related vapor intrusion mitigation resources are useful references for regulators and project managers evaluating mitigation technology selection, design, implementation, operation, maintenance, and review.

Schedule, Cost, and Transaction Risk

For developers, lenders, municipalities, and property owners, the practical impact is not only environmental risk; it is financial and schedule risk. Vapor intrusion can slow or complicate the schedule of a redevelopment project. Additional sampling may require seasonal events, access agreements, utility surveys, sub-slab sampling, indoor air sampling, or soil gas point installation. If a regulator requests more data after design completion, the team may need to revise foundation plans, mechanical systems, tenant layouts, or occupancy assumptions. In the worst cases, vapor intrusion issues may come to light during financing, permitting, tenant fit-out, or closing, adding complexity to a project when timeline flexibility is most rigid.

Why Brownfield Projects Deserve Early VI Review

The consequences can be especially acute for brownfield redevelopment. Many sites that are attractive for housing, mixed-use projects, schools, health care facilities, or commercial reuse are former industrial or commercial properties with nearby solvent or petroleum histories. EPA’s brownfields vapor intrusion tools are aimed at redevelopment stakeholders precisely because vapor intrusion can affect land revitalization decisions. The lesson for project teams is simple: the vapor intrusion possibility should be evaluated early enough to influence design, not late enough to become a change order. PPM also supports communities and project teams through brownfield redevelopment and brownfield grant applications and management when vapor intrusion concerns affect reuse planning.

Mitigation Options Are Established, But Context Matters

Vapor intrusion mitigation methods are well established, but they need to be matched to the building, contaminant, construction phase, and regulatory setting. For new construction, the most efficient approach is often a preemptive design consisting of vapor barriers, passive venting layers, sealed penetrations, and the ability to convert passive systems to active sub-slab depressurization if monitoring indicates the need. In existing buildings, active sub-slab depressurization is common, a concept familiar to radon mitigation, using piping and fans to create negative pressure beneath the slab and vent vapors safely above the roofline. Other tools may include sub-membrane depressurization for crawl spaces, sealing of preferential pathways, HVAC balancing, indoor air treatment in limited cases, and institutional controls requiring operation, maintenance, inspection, and reporting. For examples of related project work, see PPM’s representative projects on vapor intrusion assessment and remediation and brownfield vapor intrusion assessment and remediation.

Why Early Planning Controls Cost

Cost considerations vary widely, which is why early planning within a project matters. The cost difference between proactive and reactive vapor mitigation can be significant. For new construction, incorporating vapor controls during design is usually the most economical approach. A vapor barrier, venting layer, sealed penetrations, and passive piping can often be coordinated with the foundation and mechanical design before the slab is poured. Even if an active system is not immediately required, designing a passive system that can later be converted to active depressurization gives the owner flexibility at relatively low incremental cost. By contrast, retrofitting an existing or newly completed building can be far more expensive and disruptive. Long-term costs also matter: fans need power, components require inspection, alarms and telemetry may be necessary, and regulators may expect periodic monitoring with reporting. The cheapest strategy on bid day is not always the cheapest strategy over the life of an impacted project site.

Build Vapor Intrusion Into the Conceptual Site Model

The most efficient approach is to integrate vapor intrusion into the project’s conceptual site model from the beginning. That means looking beyond property lines, reviewing historical uses, identifying volatile chemicals of concern, evaluating groundwater and soil gas data, accurately mapping utilities and preferential pathways, and considering the future building plans—not just the current site structures. Where uncertainty remains, teams should decide whether to collect more data, preemptively design for mitigation, or both. For many redevelopment projects, a well-designed mitigation system can be less expensive than prolonged investigation and delay.

Assign Ownership Before Options Narrow

Vapor intrusion continues to be underestimated because evaluating and addressing it sits between project disciplines: environmental science, building design, construction, regulatory compliance, real estate finance, and risk management. No single party is responsible for identifying and addressing the issue unless the project team assigns ownership early. But when vapor intrusion is addressed proactively, it can be incorporated into the project design. The risk is not that vapor intrusion is impossible to solve. The risk is that it is discovered too late, after budgets and designs have hardened and options have narrowed. This is why vapor intrusion belongs at the front end of redevelopment planning—not as an afterthought.

The points made here hopefully demonstrate the complexity and planning that are important to evaluating and mitigating vapor intrusion issues early in the process. Feel free to reach out to me and leverage the PPM vapor intrusion team to address your vapor intrusion issues so that you can be successful at your development or redevelopment efforts. Reach out to me any time at ben.lightsey@ppmco.com.

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