Department of Health Seal

TGM for the Implementation of the Hawai'i State Contingency Plan
Section 7.7


7.7.1 Determining When to Collect Indoor Air Samples

Although counterintuitive, testing of indoor air to identify and evaluate potential vapor intrusion concerns is fraught with potential error and generally discouraged except in cases where subslab soil vapor data indicate a clear threat to indoor air (see HDOH, 2016). This is due to the common presence of the same suite of targeted VOCs in soil vapor in indoor air from sources within or outside of the building (e.g., USEPA 2011e).

As noted in Table 7-2, background levels of VOCs in indoor air from indoor sources exceed conservative action levels for many common chemicals. Correlation of indoor air data with subsurface, soil vapor data can therefore be difficult if not impossible if the concentration of VOCs identified in indoor air falls within the range of anticipated background. Although precautionary measures could be taken to mitigate potential vapor intrusion (e.g., sealing of floors, improved ventilation, etc.), as a general rule a home or building should not be flagged for potential vapor intrusion hazards unless this is supported by multiple lines of evidence, including indoor air data well above anticipated, background levels. This is discussed further in Section 7.14.2.

The presence and concentration of contaminants in indoor air is influenced by several factors, including the following: (1) the input rate of the contaminant from the source, (2) degree of ventilation and air exchange in the building, and (3) the input rate of other sources within or near the building (i.e., from outdoor air and not the subsurface). Assessments of vapor intrusion should consider the following factors:

Table 7-2 Comparison of HEER Indoor Air Action Levels to Typical Indoor Air Concentrations of Common VOCs.

Volatile Chemical

1HEER Indoor Air Action Level (ug/m3)

2Range of Background Indoor Air Concentrations
















4,5Total Petroleum Hydrocarbons









  1. HDOH, 2016; residential indoor air action level noted.
  2. USEPA 2011b; Range of 50th percentile noted (<RL = less than laboratory reporting limit).
  3. Jia and Batterman 2010; urban houses.
  4. HDOH, 2016; indoor action level equal to sum of vapor-phase, TPH aliphatic and aromatic compounds (see Section 7.11).
  5. MADEP 2008; range TPH 50th to 90th percentile.
  • Indoor air sources (other than soil vapor);
  • Outdoor air sources (other than soil vapor);
  • Location and characteristics of known or suspected soil vapor source;
  • Building ventilation and air exchange rate;
  • Building materials and condition.

A number of commonly used household products contain some of the same compounds of concern as targeted in vapor intrusion investigations (e.g., TPH and BTEX from cleaners and fuels, PCE from dry-cleaned clothes, TCE and TCA from degreasing solvents, etc.,). Common sources of VOCs in indoor air include (USEPA 2011e, ITRC 2007, HDOH, 2016):

Building Materials:

  • Carpets and adhesives;
  • Composite wood products
  • Plastics;
  • Paints;
  • Sealing caulks;
  • Parts cleaning solvents;
  • Upholstery fabrics;
  • Varnishes;
  • Vinyl and linoleum floors;
  • Polyester resins and epoxies;

Home and Personal Care Products:

  • Air fresheners;
  • Air cleaners;
  • Cleaning and disinfection products;
  • Cosmetics;
  • Fuel oil and gasoline;
  • Mothballs;
  • Running automobiles, generators or lawn equipment;


  • Smoking;
  • Dry cleaning;
  • Hobbies that involve glues;
  • Newspapers;
  • Non-electric space heaters;
  • Photocopiers;
  • Stored paints and chemicals.

In some areas, especially urban centers, TPH, benzene and related contaminants associated with auto exhaust in outdoor air can also exceed conservative, indoor air action levels.

For these reasons, testing of indoor air to evaluate potential vapor intrusion impacts is generally discouraged unless concentrations of targeted chemicals in subslab soil vapor are more than one-thousand times typical indoor air concentrations for residences and two-thousand times typical indoor air concentrations for commercial/industrial buildings (assumed indoor air:subslab soil gas attenuation factors; see Sections 7.3, 7.14 and Table 7-2; see also HDOH, 2016). Indoor air sampling may also be warranted if field screening of potential vapor pathways inside of a building suggest that vapors could be impacting indoor air at levels significantly above background and the pathways are unlikely to be sealed in the absence of indoor data to support such actions (e.g., PID readings around utility gaps in floors, drains, wall sockets, etc.; see Section 7.7.2; see also MADEP 2002b, CalEPA 2011, New York State DOH 2006).

The evaluation of potential vapor intrusion hazards and decisions regarding the need for remedial actions will instead, in most cases, focus on subslab or crawl space data. Shallow soil vapor data or data for samples collected under paved areas can be taken into consideration for sites without existing buildings.

Subslab (including sub-basement) soil vapor samples should be collected for buildings with a slab-on-grade construction. For buildings with a crawl space design, shallow soil vapor samples should be collected adjacent to the building in addition to samples from the crawl space. In both cases, it is preferable that soil vapor and/or crawl space samples be collected prior to collection of indoor air samples and used to determine the need to collect the latter (see HDOH 2016). If a significant source of potential vapors is present below the building (e.g., petroleum free product on shallow groundwater) then the collection of source area soil vapor samples is also recommended (see Sections 7.6.2). Together, subslab and source area data should be reviewed to determine the need for the collection of indoor air samples.

If indoor air sampling is still desired or required, then sample collection and interpretation of data should be carried out under the direction and oversight of HDOH. Soil vapor (or crawl space) samples should be collected at the same time in order to assist in the interpretation of the indoor air data (see Section 7.14). Indoor air data should never be used as the only line of evidence for vapor intrusion.

Indoor air data should be compared to both risk-based screening levels and anticipated background concentrations. More than one round of sampling is recommended if a significant source of vapors is identified beneath a building (see Section 7.11.3). If representative concentrations of targeted VOCs fall within the range of anticipated background concentrations then active measures to address vapor intrusion are not necessary (see Section 7.14.2) , although sealing of cracks and gaps in floors should be carried out as a precautionary measure (see Section 7.14.1).

7.7.2 Indoor Air Sampling Design

Specific guidance regarding the location, duration, and frequency of indoor air sampling is provided in Section 7.11. The following factors will influence an indoor air quality sampling strategy:

Sources: All of the potential sources of indoor air contamination should be considered in developing a sampling plan. Sources can include, but are not limited to, subsurface contamination (i.e., vapor intrusion), indoor sources (i.e., use/storage of VOC containing chemicals), and outdoor background sources (i.e., VOCs in the ambient background air around a building).
Pathways: Likely or potential pathways for VOCs to enter the building air should be considered in developing a sampling strategy. Common pathways for vapor intrusion from the subsurface are cracks or utility penetrations through the slab or basement walls/floor, sumps with earthen floors, and drain pipes. Elevator shafts could also serve as pathways for vapor intrusion, although these structures tend to mimic chimneys by conducting air out of rather than into buildings, including vapors that might intrude into the bottom area of the shaft. Bathrooms, kitchens and utility rooms are often the primary entry points for intruding vapors. VOCs can also enter a building through the heating, ventilation, and air conditioning (HVAC) system intake if the ambient air contains VOCs (if a VOC source is located upwind of the HVAC intake, higher concentrations of VOCs can be introduced to the indoor air than would be expected based on the general ambient air conditions around the building). Most buildings have indoor sources of VOCs-- these should be carefully evaluated and, if possible, removed prior to sampling.
Heating, Ventilating, and Air Conditioning: Operating conditions of the HVAC system can have a significant effect on VOC concentrations over a short time frame (see Section 7.1). The concentration of VOCs will generally be lowest when the HVAC air conditioning system is operating, due to the inflow of fresh air, dilution of vapors from indoor sources and the reduction of subsurface vapor intrusion when the building is over pressurized. Indoor air quality will be lowest when the HVAC system is not operating, due to the lack of fresh air entering the building to offset VOC emissions from furniture, plastics, glue and other indoor sources as well as the potential for the building to become under-pressured due (at least in Hawai‘i) to outside wind effects that could induce an upward flow of subsurface vapors.
Occupants: The presence and activities of building occupants can have a significant effect on VOC concentrations in indoor air. For example, freshly dry-cleaned clothing can introduce PCE to the indoor air, and smoking can release a variety of VOCs including benzene. Opening of doors and windows can increase the building air exchange rate and thereby lower VOC concentrations (assuming that higher concentrations are not present in outdoor air).

To prevent, investigate, and resolve indoor air quality problems, it is important to understand the role each of these factors can play.

The indoor air sampling strategy should identify the location, duration, and frequency of indoor air sampling. As with soil vapor sampling strategy, location, duration, and frequency will be influenced by site-specific conditions and the study objective. For example, if the goal of the study is to obtain a representative estimate of the average concentration of contaminants in indoor air that could present potential risk to the occupants of a building, then several indoor air samples should be collected in various portions of the occupied space and weighted accordingly. If the goal of the study is to measure the highest concentration to which an occupant might be exposed, then sampling could be conducted in the area closest to the suspected source or pathway for contamination (MADEP 2002b).