INTRODUCTION

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Formaldehyde is a common air pollutant in homes. It is highly irritant to the mucous membranes and its effect on the eyes and upper respiratory tract is well documented (1). In the workplace, exposure to formaldehyde has been implicated in occupational asthma (2) and, recently, domestic exposure has been associated with lower respiratory tract symptoms and disease in both children (3) and adults (6, 7). Specific mechanisms to account for these findings have not yet been identified; however, it is possible that formaldehyde may have a direct toxic effect on the respiratory epithelium, inducing inflammatory reactions (8).

Inflammatory changes are observed in the upper airways after acute low level exposure to formaldehyde (9), and damage to the lower airways is reported after exposure to high levels (5 to 30 ppm) (1). The inflammatory effects of chronic low level exposure on the lower airways, however, have not been well researched. The aim of the present study was to investigate the possible inflammatory effects of formaldehyde concentrations typically encountered in the domestic environment. Nitric oxide (NO) measured from the exhaled breath of children was used as a marker of airway inflammation.

	METHODS

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Subjects

In order to measure airway responses that are not modified by existing inflammation we recruited healthy children for this study. A modified respiratory health questionnaire (10) and household inventory (11) was distributed through a number of local primary schools, and only children with no current, or history of, upper or lower respiratory tract disease were included in the study. Two hundred twenty-four children (116 girls) were involved in the study. The median age was 9.5 yr and ranged from 6 to 13 yr.

All homes of the participating children were monitored for formaldehyde using a passive monitoring technique. Airborne formaldehyde levels were measured in the child's bedroom and the main living area of the house over a 3- to 4-d period. Each child made one visit to the children's hospital for a respiratory assessment. This included spirometry, a skin prick test, and eNO measurements. The protocol was approved by both the King Edward Memorial and Princess Margaret Hospitals Ethics Committee and Murdoch University Human Research Ethics Committee, and written informed consent was obtained from the parents.

Protocol

Indoor levels of formaldehyde were measured with passive monitors using 2,4-dinitrophenylhydrazine (DNPH) as a capture medium (12). The sensitivity of the monitors is 1 ppb in a 24-h sample (12). Formaldehyde concentrations were analyzed using high performance liquid chromatography (Shimadzu LC-10AD; Shimadzu, Kyoto, Japan) and were recorded as a time-weighted average (TWA) for the sampling period.

Exhaled NO was measured during a single-breath exhalation using a fast response (0.02 s) chemiluminescence analyzer (NOA 280; Sievers Instruments Inc., Boulder, CO). Measurements were based on the technique described by Silkoff and colleagues (13). While seated the children breathed through a mouthpiece attached to a one-way valve. The valve had two sampling ports near the mouthpiece. Nitric oxide was sampled directly into the analyzer through a teflon side-arm tube attached to one of the sampling ports. Exhalation (mouth) pressure was measured by a pressure transducer in the analyzer via the second sampling port. Measurements were made for each child using a mouth pressure of 15 cm H₂O corresponding to an expiratory flow of 75 ml/s. After inhalation to TLC the children immediately exhaled into the mouthpiece. Mouth pressure was displayed on a computer screen as a prompt for the children to maintain a steady flow. Nitric oxide values were recorded as the plateau at the last part of the exhalation. All children completed at least three measurements, and NO concentrations were recorded as the average of three plateaus that varied by less than 10%.

All children underwent spirometry according to ATS guidelines (14). Skin prick test (SPT) reactions were measured for seven common allergens: egg, cow's milk, cat hair, dog hair, grass mix, Dermatophagoides pteronyssinus, and Alternaria tenuis (Miles Laboratories, Inc., Elkhart, IN) Histamine hydrochloride (1.0 mg/ml) and a saline solution were used as positive and negative controls. Weal sizes were measured after 15 min as the diameter of the long axis of the raised area. Results were interpreted in relation to the size of the reaction to the histamine, and only a weal of equal or greater diameter was accepted as a positive result.

Statistical Analysis

Exhaled NO levels were skewed to the right so values were transformed to their natural logarithm (ln) to achieve a normal distribution. Formaldehyde concentrations were grouped as high or low using a cutoff level of 50 ppb. Bivariate analyses (Student's t test, one-way analysis of variance, and correlations) were used to determine the effect of formaldehyde levels and other housing factors (obtained from the housing questionnaire) on both ln (eNO) levels and spirometric variables. A multivariate linear regression model was then performed with variables that were found to have at least a marginally significant relationship (p < 0.1) with ln (eNO) in the bivariate analyses. The model included child's age and atopic status (determined by the skin prick test) as both have previously been associated with raised eNO levels (15). Exhaled NO values are reported as the geometric mean with 95% CI. Other variables are expressed as the arithmetic mean with 95% CI. All analyses were done using SPSS 8.0 (SPSS Inc., Chicago, IL).

	RESULTS

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Formaldehyde levels measured in the bedroom and living room were combined to provide an average concentration for each home. There was no effect of formaldehyde levels on FVC or FEV₁. There was, however, significantly higher levels of eNO measured from children living in homes with average formaldehyde levels greater than 50 ppb (p = 0.02). Exhaled NO levels were 15.5 ppb (10.5 to 22.9 ppb) for children from homes with formaldehyde concentrations  50 ppb and 8.7 ppb (7.9 to 9.6 ppb) for children from homes with formaldehyde concentrations < 50 ppb (Figure 1). This remained significant after controlling for all other variables in the multiple regression model (p = 0.002).

View larger version (12K): [in this window] [in a new window]   Figure 1.   Average eNO levels (ln-transformed data) for children living in homes with average formaldehyde concentrations of < 50 ppb or  50 ppb. The median is the line in the box, whereas box limits represent 25th and 75th percentiles. The whiskers extend to the 10th and 90th percentiles. There was a significant difference between the two groups of children (*p = 0.002).

	DISCUSSION

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Formaldehyde is an intensely irritating gas that primarily affects the eyes and upper airways. Chronic exposure to domestic levels of formaldehyde, however, has been associated with nocturnal chest tightness in adults (6), decreased peak expiratory flow (3), bronchial hyperreactivity (4), increased asthma (3), and asthmalike symptoms (4, 5) in children. We have demonstrated that exposure to formaldehyde, at levels typically found in homes, is associated with raised eNO levels in healthy children, suggesting that chronic exposure to low levels of formaldehyde may induce an inflammatory response in the airways.

In the present study we used eNO as a marker of lower airway inflammation. There is strong evidence to support the assumption that eNO reflects airway inflammation (16), and the technique we used excludes NO originating from the upper airway (13). Endogenous NO is synthesized from L-arginine by isoenzymes of NO synthetase (NOS). In inflammatory disease NO is increased by an upregulation of the inducible form of NOS (iNOS) (16). Nitric oxide derived from other isoforms of NOS (cNOS) is involved in the physiologic regulation of airway function (16). Although the source of the elevated eNO observed in this study is not known, we hypothesize that formaldehyde causes inflammation and induction of iNOS by inflammatory cytokines.

The majority of inhaled formaldehyde is absorbed in the upper respiratory tract and histopathologic changes in the nasal cavity are well documented. Inflammatory cellular changes have been observed in the upper respiratory tract of both animals (19) and humans (9, 20). Indeed, acute exposure to relatively low levels of formaldehyde (0.4 ppm) is sufficient to induce eosinophilia in the nasal passages of healthy persons (9).

Lower airway inflammation has not been observed in mice or guinea pigs after formaldehyde exposure (8, 21). However, formaldehyde-induced lesions of the respiratory tract have been detected in the trachea and major bronchi of monkeys (19), suggesting species differences in the lower airway responses to formaldehyde. In humans, lower airway and pulmonary effects are observed after exposure to formaldehyde at levels greater than 5 ppm (2). Data on the inflammatory effects of chronic low level formaldehyde exposure are sparse, although Weislander and colleagues (7) found that blood eosinophils were significantly increased in adults living in newly painted dwellings, which in turn was associated with increased levels of formaldehyde and other volatile organic compounds (VOCs). Average formaldehyde levels for newly painted houses in their study were less than 50 ppb. Our data suggest that, in children, inflammation may extend beyond the upper airways into the lower airways as a result of chronic exposure to domestic levels of formaldehyde.

Formaldehyde exposure has been shown to modulate the immune response to allergens. For example, formaldehyde enhances sensitization to inhaled allergens in guinea pigs (21) and mice (8). IgE-mediated sensitization to formaldehyde has been found in children exposed to formaldehyde in schools (22), and residential levels of formaldehyde have been associated with atopy (5). A specific immunologic hypersensitivity to formaldehyde has only rarely been demonstrated in occupationally exposed adults (2). It is possible, therefore, that the reported immune responses may result from airway epithelial damage causing increased airway permeability and other inflammatory changes that could allow easier penetration of inhaled allergens to cells of the immune system (23).

The association between formaldehyde concentrations and eNO levels in the present study occurred in children with no previous airway damage and was independent of atopy. The apparent proinflammatory activity of formaldehyde, demonstrated in this study, could have a number of important consequences and might explain some of the observed associations between formaldehyde exposure, respiratory morbidity, and immunolgic responses. Further research is required to confirm these results and explore their implications.