A coupled sensor-spectrophotometric device for continuous measurement of formaldehyde in indoor environments

Abstract

Despite long-standing awareness of adverse health effects associated with chronic human exposure to formaldehyde, this hazardous air pollutant remains a challenge to measure in indoor environments. Traditional analytical techniques evaluate formaldehyde concentrations over several hours to several days in a single location in a residence, making it difficult to characterize daily temporal and spatial variation in human exposure to formaldehyde. There is a need for portable, easy-to-use devices that are specific and sensitive to gas-phase formaldehyde over short sampling periods so that dynamic processes governing formaldehyde fate, transport, and potential remediation in indoor environments may be studied more effectively. A recently developed device couples a chemical sensor element with spectrophotometric analysis for detection and quantification of part per billion (ppbv) gas-phase formaldehyde concentrations. This study established the ability of the coupled sensor-spectrophotometric device (CSSD) to report formaldehyde concentrations accurately and continuously on a 30-min sampling cycle at low ppbv concentrations previously untested for this device in a laboratory setting. Determination of the method detection limit (MDL), based on 40 samples each at test concentrations of 5 and 10 ppbv, was found to be 1.9 and 2.0 ppbv, respectively. Performance of the CSSD was compared with the dinitrophenylhydrazine (DNPH) derivatization method for formaldehyde concentrations ranging from 5–50 ppbv, and a linear relationship with a coefficient of determination of 0.983 was found between these two analytical techniques. The CSSD was also used to monitor indoor formaldehyde concentrations in two manufactured homes. During this time, formaldehyde concentrations varied from below detection limit to 65 ppbv and were above the US National Institute for Occupational Safety and Health (NIOSH) recommended exposure limit (REL) of 16 ppbv, which is also the exposure limit value now adopted by the US Federal Emergency Management Agency (FEMA) to procure manufactured housing, 80% and 100% of the time, respectively.

INTRODUCTION

Formaldehyde is one of the most pervasive and consistent pollutants in indoor environments, owing to a wide range of building materials and consumer products present indoors that emit formaldehyde and to a variety of chemical reactions that occur in indoor environments that generate formaldehyde. Chronic exposure to formaldehyde is often of greatest concern in indoor environments, where concentrations may be four to ten times greater than outdoor concentrations,1 and such exposure is associated with numerous hazardous health end points, including decreased pulmonary function, sensory and respiratory irritation, respiratory tract pathology, increased asthma incidence and prevalence, and increased damage to immune systems.2 Furthermore, long-term exposure to formaldehyde is associated with lymphohematopoietic and nasopharyngeal cancers; thus, formaldehyde has been classified as a known human carcinogen by the International Agency for Research on Cancer (IARC) in 20063 and more recently in 2010 and 2011 by the US Environmental Protection Agency2 and US Department of Health and Human Services,4 respectively.

Increased understanding of the health risks associated with human exposure to formaldehyde at concentrations observed in indoor environments, which may range from 4–110 ppbv,5 and a growing trend to lower chronic recommended exposure limits have accelerated the need for analytical techniques capable of measuring ppbv formaldehyde concentrations dynamically.

Numerous international organizations and national- or state-level governmental agencies have developed a range of recommended exposure limits for formaldehyde. For instance, following reports from the Agency for Toxic Substances and Disease Registry and the US Centers for Disease Control of high formaldehyde concentrations in manufactured (prefabricated) housing provided by the United States Federal Emergency Management Agency (FEMA) to families who were displaced by hurricanes Katrina and Rita in 2005, FEMA implemented procurement guidelines for manufacturers of mobile and manufactured housing. The guidelines require formaldehyde in all manufactured housing units to be less than the recommended exposure limit (REL) set by the National Institute for Occupational Safety and Health (NIOSH) of 16 ppbv over an 8-h period.6 On the global scale, the World Health Organization has set a 30-min and chronic REL for formaldehyde of 81 ppbv,7 which has been under consideration for 25 years.8 A summary of RELs for non-cancer effects from various agencies in the United States of America and around the world is presented in Table 1.

Table 1 Recommended exposure limits and threshold limit values for formaldehyde with respect to non-cancer health effects

A different REL has been developed for formaldehyde to take into consideration cancer effects associated with formaldehyde exposure. Only in exceptional cases would the guideline value of 1.63 ppbv for 1 in 100,000 lifetime cancer risk set by the CA OEHHA,9 which is less than typical outdoor levels,5 be attainable in residences. Current analytical techniques must continue to advance to monitor indoor formaldehyde concentrations down to this low level.

Traditional sampling methods for formaldehyde in indoor environments rely on pre-concentration and derivatization phases during sampling, after which the sample must undergo further preparation to be analyzed by the appropriate chromatographic or spectroscopic technique. Numerous reviews thoroughly describe in situ and derivatization-based sampling methods and analytical techniques or directly compare in greater detail two or more existing techniques.10, 11, 12, 13, 14 Of the many techniques outlined in these studies, the dinitrophenylhydrazine (DNPH) method has become the accepted international standard procedure for analysis of formaldehyde in indoor air by the International Organization for Standardization, and is described within the EPA method TO-11A15 and the ASTM D519716 for the determination of aldehydes in air. This and other derivatization methods suffer from long sampling times, typically several hours to several days, thus precluding the study of dynamic processes. In response to a need for shorter sampling times, Martos and Pawliszyn17 developed a method that employs derivatization of formaldehyde with o-(2,3,4,5,6-pentafluorobenzyl) hydroxylamine hydrochloride (PFBHA) on solid-phase microextraction (SPME) fibers followed by thermal desorption onto a gas chromatograph and detection by flame ionization (FID) or electron capture (ECD). Although sampling times are reduced to the range of 1–10 min, preparation of the SPME fiber before sampling and subsequent GC-FID or ECD analysis extends the overall sampling time. Furthermore, this method is entirely manual, making continuous, dynamic sampling labor- and time-intensive. Accordingly, SPME-based formaldehyde monitoring has not achieved widespread use, and the need persists for sensor technology that can perform real-time measurement of formaldehyde and that is easy to transport to and use in the field.

Development of sensor-based sampling methods and analytical techniques is growing rapidly with increasing demand for such technology. Traditional real-time sensor technology employing the Hantzsch method, which relies on the quantitative transfer of formaldehyde from the gas phase to the liquid phase,18 is labor- and resource-intensive.14 Several commercially available real-time formaldehyde sensors that use either electrochemical or photoelectric photometry technologies were recently tested by the National Research Council of Canada under conditions similar to those used in this study and demonstrated good linearity, stability, and repeatability.19, 20 However, method detection limits (MDLs) were not evaluated for any of these devices nor was performance evaluated at concentrations <10 ppbv. Until recently, major shortcomings of real-time formaldehyde sensor technology have been high detection limits,14 sensitivity to relative humidity and temperature, and cost. A new chemical sensor element21 has demonstrated the ability to reliably report low formaldehyde concentrations in residential environments.22 Sensor technology of this kind can be readily incorporated into a small, portable device. This study investigated performance, in the laboratory, of one such coupled sensor-spectrophotometric device (CSSD) at low ppbv concentrations, particularly those below 10 ppbv, relative to the standard DNPH derivatization method and evaluated the MDL at two test concentrations for four unique devices of the same design. The particular CSSD was also selected because the cross-sensitivity of device measurements to numerous, common, indoor air pollutants has been previously addressed in the literature.22, 23 These authors found that interference with formaldehyde measurement is negligible at concentrations of tested indoor air pollutants below 1 ppmv, which is well above the typical concentrations anticipated for these pollutants in residential settings.

The development of new formaldehyde sampling methods and analytical techniques well suited for dynamic indoor environments is of critical importance. Currently, when residential formaldehyde concentrations are reported, they represent an average value weighted over a time span of several hours to several days. For example, in two of the most recent, broad-based human exposure assessments to include formaldehyde—the US EPA National Human Exposure Assessment Survey (NHEXAS)24 conducted in 189 homes in Arizona and the Relationships of Indoor, Outdoor, and Personal Air (RIOPA) study conducted in 311 homes in three urban centers in the United States of America25—the formaldehyde concentrations were reported as time-weighted averages taken over a 6–7-day period and a 48-h sampling period, respectively. The median formaldehyde concentration in the NHEXAS study and the mean formaldehyde concentration in the RIOPA study were both 17 ppbv and 21.6 ppbv, respectively. However, it is important to consider upper limit concentrations as well as central limit tendencies, especially in light of evidence that suggests peak exposure dose metrics are stronger than cumulative exposure dose metrics when evaluating causal associations between formaldehyde exposure and risk for certain lymphohematopoietic cancers.26 Monitoring formaldehyde concentrations over shorter time scales would be valuable for developing a better understanding of temporal concentration variations. Small, portable, rugged devices also make it possible to monitor formaldehyde in several different locations in a home. This information would not only allow for better characterization of daily human exposure to formaldehyde, but it could also inform the timely application and installation of potential treatment strategies.

Manufactured homes are expected to exhibit higher levels of formaldehyde relative to site-built residences because of the building materials used in their manufacture.27 Housing developments in the state of Texas known as “colonias” are dominated by single-family manufactured homes. These communities frequently lack safe and healthy housing,28 and knowledge of indoor environmental quality in these homes is completely absent. This study undertook to investigate the performance of the recently developed CSSD in the field by measuring formaldehyde concentrations in several manufactured homes in a colonia outside of San Marcos, Texas, USA.

METHODS

CSSD Calibration

The sensor element used in this study was designed for spectrophotometric analysis and consists of one chemically coated porous glass sheet adjacent to one non-coated glass sheet. The chemically coated glass sheet reacts with formaldehyde, producing a change in color on the glass, and when the absorbance at 410 nm is measured through both the coated and the un-coated glass sheet, the difference in absorbance is proportional to the formaldehyde concentration. Development of this sensor technology has been fully described elsewhere.21 The CSSD (Shinyei Technology; Kobe, Japan; Formaldehyde Multi-Mode Monitor) is a battery-operated unit, housing both the chemical sensor element and the spectrophotometric equipment necessary to evaluate the sensor absorbance on a semi-continuous basis. Temperature and relative humidity are also monitored on a 30-min cycle. The data is stored in the CSSD until it can be downloaded to an available computer.

The experimental setup used to calibrate the CSSDs is illustrated in Figure 1. A total of four CSSDs, each equipped with an individual chemical sensor element, were placed in a single, 10 l, stainless steel sampling chamber (Eagle Stainless, Warminster, PA, USA; CTH-24) into which was fed a single stream of nitrogen gas with a known formaldehyde concentration. The formaldehyde concentrations tested were 5, 10, 13, 25, and 50 ppbv. A zero concentration case was also tested, during which time only zero-grade (99.998%) nitrogen gas was fed to the sampling chamber. Each concentration was tested for 4–6 h, corresponding to 8–12 sensor absorbance readings per monitor. The temperature in the sampling chamber during sampling was 20±1 °C. The relative humidity was maintained at 50±2% to reflect a level of relative humidity that would be encountered in a typical residential setting.

Figure 1
figure1

Schematic of the experimental setup used to generate formaldehyde and test CSSDs.

Formaldehyde gas was generated using a Kin-Tek standard gas generator (Kin-Tek, LaMarque, TX, USA; model 491MB), which is equipped with a temperature-controlled oven to incubate an NIST-certified formaldehyde permeation tube at a specified temperature to maintain the certified emission rate. The effluent formaldehyde concentration from the standard gas generator can then be adjusted using the internal mass flow controller to change the flow of nitrogen gas passed over the permeation source. As the flow of nitrogen increases, the concentration of the formaldehyde-laden gas stream is diluted. Two permeation sources (Kin-Tek, LaMarque, TX, USA; 33896 and 32684) with different certified emission rates were used separately to achieve the full range of concentrations tested. The permeation source with the lower emission rate was used to achieve formaldehyde concentrations below 25 ppbv, whereas the permeation source with the higher emission rate was used to achieve formaldehyde concentrations of 25 ppbv and above.

To test the performance of the formaldehyde monitors under conditions similar to actual environments, the formaldehyde gas stream was humidified to achieve a constant relative humidity of 50%. To accomplish this, nitrogen gas was regulated by a mass flow controller (Stamford, CT, USA; Omega Engineering; FMA5514ST) before being bubbled through an Erlenmeyer flask containing deionized water to humidify the gas stream and subsequently combined with the formaldehyde-enriched effluent from the Kin-Tek to achieve a total flow rate that corresponded to a given target formaldehyde concentration.

While continuous CSSD measurements were taken, formaldehyde samples were simultaneously collected for analysis using the DNPH derivatization method. In accordance with the EPA TO-11A15 and ASTM D519716 standard procedures, DNPH-coated sorbent tubes (Eighty Four, PA, USA; SKC; 226-119), connected to an air sampling pump, actively sampled the effluent leaving the stainless steel chamber at a flow rate of 493 ml/min over the same period of time (4–6 h) that the CSSDs were exposed to the same concentrations. Following sample collection, the sample cartridge was eluted with acetonitrile and analyzed directly with high-performance liquid chromatography (Milford, MA, USA; Waters; Model 486) using a modified EPA TO-11A procedure. The eluent used was a 65/35 percent by volume acetonitrile/water solution, which was pumped (Brea, CA, USA; Beckman Instruments; 1106) at a constant flow rate of 1.5 ml/min through two 5 μm Reverse Phase C18 columns connected in series. The first column was 250 mm in length (Santa Clara, CA, USA; Agilent Technologies; Zorbaz ODS) and the second column was 150 mm in length (St. Louis, MO, USA; Supelco Analytical; LC18).

Manufactured Home Field Measurement

One CSSD was placed in each of two manufactured homes and was undisturbed for 5 days while they continuously measured formaldehyde concentrations in these homes. The homes were similar in size (500 m3 in total volume), layout (three-bedroom homes of 140 m2 each), and age (each over 10 years old), and they were occupied during the entire field measurement period. New sensor elements were installed in the CSSDs at the start of field sampling in the homes, and the CSSDs were placed in a common room (not a bedroom or the kitchen) at a height of 1.5 m above floor level. The CSSDs were placed so as to avoid any direct contact with known sources of formaldehyde emissions (e.g. on top of cabinetry made from medium-density fiberboard). After 5 days, the CSSDs were retrieved.

RESULTS AND DISCUSSION

CSSD Calibration

Four CSSDs were used to measure six different formaldehyde concentrations continuously for 4–6 h, taking an absorbance reading every 30 minutes. Performance by the four CSSDs was evaluated for equivalence using a two one-sided test (TOST) procedure. For this analysis, a (1–2α) 100% confidence interval was constructed,29, 30 where α=0.05 (just as with null hypothesis difference analysis) and z1−α=1.645. The null hypothesis for this analysis was that the CSSDs differ by at least Δ=±10%. Thus, any two CSSDs were considered equivalent if the 90% confidence interval calculated for the difference between the two CSSDs was contained within the interval±10%.

According to the TOST analysis, all four CSSDs were found to behave equivalently, demonstrating the precision of the devices. The data from all CSSDs were then pooled to determine an average concentration over the given sampling period. Formaldehyde concentrations determined using the CSSDs were plotted versus formaldehyde concentrations determined using the DNPH derivatization method and presented in Figure 2. The statistical analysis showed very strong agreement between the two analytical techniques with a coefficient of determination of 0.983. This result is important because it demonstrates the ability of the CSSD to closely match the performance of the DNPH standard procedure for formaldehyde monitoring, which is currently considered the most accurate formaldehyde detection method. Based on the slope of the linear curve fit (Figure 2), CSSD measurements tend to be slightly higher than DNPH measurements. Of particular note, manufacturers of the CSSDs evaluated in this study have been yet unable to test performance at concentrations below 10 ppbv and currently report a detection limit of 20 ppbv; therefore, this study has provided new and valuable insight into the performance of a CSSD.

Figure 2
figure2

Correlation between reported formaldehyde concentrations from CSSDs and the DNPH derivatization method.

Devices such as these, which base detection on measurement of optical absorbance, may be subject to solvatochromic effects. In the laboratory setting for this study, obtaining stable formaldehyde measurements at relative humidity levels below 30% was found to be challenging. However, for the relative humidity level selected for this study (50% RH), the CSSDs were found to be robust with respect to formaldehyde detection and quantification based on the agreement observed between simultaneous CSSD and DNPH measurements. Robust performance by the formaldehyde sensor element at 50% RH was also observed by Tokumitsu et al.23 and Maruo et al.22

Evaluation of CSSD MDL

The MDL associated with 40 measurements taken at a 5-ppbv level by the four CSSDs was estimated as the product of the SD of the 40 replicate samples at a 5-ppbv level (SD5 ppb=0.947) and the one-tailed t-statistic for n=39 degrees of freedom at the 95% confidence level (t(n=39,α=0.05)=2.042). The estimated MDL was 1.9 ppbv.

To test the robustness of the MDL estimate, the same procedure was applied at the 10-ppbv level. With SD10 ppb=0.982 and t(n=39,α=0.05)=2.042, the estimated MDL was 2.0 ppbv, suggesting that this evaluation is robust and not dependent on the initial test concentration.

Manufactured Home Field Measurement

The formaldehyde concentration data presented in Figure 3 were obtained from continuous monitoring of two manufactured homes over the course of 5 days, with measurements recorded every 30 min. Concentrations in both homes are typically, in the case of home 2, and exclusively, in the case of home 1, above the NIOSH REL. In fact, formaldehyde concentrations in manufactured home 1 are above the NIOSH REL for the entire sampling period. In manufactured home 2, formaldehyde concentrations are above the NIOSH REL 80% of the time. Even so, it is significant to consider that over the course of 5 days of monitoring, the formaldehyde concentrations show considerable variability. The time-weighted average formaldehyde concentrations evaluated over 5 days in manufactured homes 1 and 2 are 34.2 and 22.4 ppbv with SDs of ±6.5 and ±10.7 ppbv, respectively. Formaldehyde concentrations in manufactured home 1 ranged from 17–53 ppbv, whereas those in manufactured home 2 ranged from below the detection limit to 65 ppbv. It was observed that manufactured home 1 contained more home furnishings, wood-paneled walls, and composite wood products than did manufactured home 2, which might provide some explanation for the higher average formaldehyde concentration. At the same time, occupants in manufactured home 2 cooked meals more frequently and for longer duration than occupants of manufactured home 1, and whether cooking events influence indoor formaldehyde concentrations would be worth further study.

Figure 3
figure3

Continuous formaldehyde sampling for 5 days in two manufactured homes in a colonia outside San Marcos, Texas, USA. Data points marked with “X” are below the detection limit.

These results could have important implications for the ability to conduct dynamic formaldehyde monitoring in actual residential, commercial, and occupational environments, where previously only single, time-averaged data points could be collected. Continuous characterization of indoor formaldehyde concentrations, as has been presented here, makes it possible to identify internal or external environmental parameters, including relative humidity and temperature indoors and outdoors, or specific activity patterns that may influence or exacerbate human exposure to formaldehyde indoors. Devices such as the one evaluated in this study enable researchers to quickly develop rich datasets of temporal and spatial variation in formaldehyde concentrations in a large number of homes. Taken together with housing characteristics and occupant time–activity patterns, strategies to reduce human exposure to formaldehyde, such as modifying a certain behavior or removing a specific source, can be targeted and effective.

This analytical technique also makes it possible to evaluate treatment strategies for their performance on a dynamic basis. During development of treatment materials or strategies to reduce formaldehyde exposure in indoor environments, knowledge of real-time formaldehyde concentrations upstream and downstream of a particular treatment strategy can shed light on the removal mechanisms at work, as well as the ability of a given treatment strategy to maintain formaldehyde levels below acute and chronic RELs. Knowledge of spatial variability in indoor formaldehyde concentrations would also make it possible to target specific placement of a treatment material. Similarly, it would be possible to identify specific timing or frequency of a treatment material’s use, once greater understanding of temporal variability in indoor formaldehyde concentrations is available.

This study improved understanding of a newly developed CSSD capable of continuous measurement of gas-phase formaldehyde concentrations. The MDL, in a laboratory setting, for the new instrument determined in this study was shown to be competitive with the widely accepted standard method of DNPH derivatization. Furthermore, the CSSD requires only 2 h or less to report an initial 30-min average formaldehyde concentration without additional sophisticated analysis in the laboratory on the part of the researcher. This combined sampling method and analytical technique influences the ability of homeowners, regulators, public health investigators, and researchers to assess temporal and spatial variability of formaldehyde concentrations within a home and across a wide range of indoor environments. This capability is especially important when investigating the relative impacts of formaldehyde treatment strategies. The application of the CSSD is not intended to replace the internationally accepted DNPH derivatization method, or other such well-established methods. However, the CSSD offers regulators, scientists, and engineers the ability to complement data from traditional analytical methods by revealing more finely resolved spatial and temporal trends in formaldehyde concentrations that inform both policy-level decisions, as well as design of appropriate treatment technology.

Although this study shows great promise in measuring sub-10 ppb levels of formaldehyde in the laboratory, a limitation of this study was that DNPH samples were not obtained during field sampling. An additional study, such as the recent investigation of CSSD performance with concurrent DNPH measurements in unoccupied test homes,31 is needed to obtain field correlations between the CSSD device and the DNPH derivatization method. Furthermore, consistency of results in the field between CSSD units and individual sensors, particularly between sensor batches, and consistency of CSSD measurements at relative humidity levels that are below 30% RH await further study.

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Acknowledgements

We thank Matrix Analytical Laboratories for their assistance in completing analysis of DNPH samples and the Indoor Air Quality Group at the National Institute of Standards and Technology for loan of equipment. EMC and this work were supported by US Environmental Protection Agency STAR Fellowship program and the American Society of Heating, Refrigerating, and Air-conditioning Engineers Grant-In-Aid. We would also like to thank the community members of Cottonwood Creek for the opportunity to carry out sampling in the field.

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Correspondence to Ellison M Carter.

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Keywords

  • coupled sensor-spectrophotometric device
  • formaldehyde
  • manufactured housing

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