The effect of humidity on formaldehyde emission parameters of a medium-density fiberboard: Experimental observations and correlations

Highlights

• Initial emittable formaldehyde concentration (C₀) was greatly influenced by humidity.

• Empirical linear relations between C₀ and humidity were obtained for a fiberboard.

• Formaldehyde emission was in consistent and instantaneous change with humidity.

Abstract

Initial emittable concentration (C₀), diffusion coefficient (D_m) and partition coefficient (K) are the three key emission parameters determining formaldehyde emissions from “dry” building materials. Previous studies of humidity effect on formaldehyde emissions were mainly focused on the analysis of steady-state emission rates or concentrations, whereas humidity effects on emission parameters were seldom discussed. In this study, we measured formaldehyde emissions of a medium-density fiberboard in a dynamic small-scale environmental chamber at the relative humidity (RH) of 20%, 30%, 50% and 80%, respectively. Emission parameters were estimated and the effects of humidity were analyzed. C₀ was the most sensitive parameter influenced by humidity, which increased 2.97-fold when RH changed from 20% to 80%. Empirical positive linear relations between C₀ and RH, C₀ and absolute humidity (AH) were identified. Differences of D_m and K between each humidity scenarios were within 11% and 17%, suggesting the negligible impacts of humidity on them. Moreover, the reversible and simultaneous responses of formaldehyde emissions to humidity changes were exhibited in experiments. Possible mechanisms of humidity effect on formaldehyde emission were hydrolysis of resins or polymers and adsorption competitions between formaldehyde and water molecules. The conclusions and empirical relations obtained in this study would be useful in understanding emission parameters at different humidity conditions.

Introduction

Formaldehyde is one of the most ubiquitous and priority pollutants indoors [1]. Numerous studies have verified that short-term exposure to formaldehyde could cause eye, nose and throat irritation [2], [3], [4]. Respiratory illness and even cancer could be induced because of the chronic exposure [5], [6], [7]. Building materials such as furniture and wood-based panel products are the most dominating formaldehyde emission sources indoors [8]. Field study has shown that formaldehyde emissions from some E1 wood-based boards could last as long as seven years [9]. Consequently, understanding the emission characteristics of these wood-based materials is meaningful to effective source control in actual buildings.

Initial emittable concentration (C₀), diffusion coefficient (D_m) and partition coefficient (K) are the three key emission parameters of the diffusion mass transfer models [10], [11]. Time-varying volatile organic compounds (VOCs) and formaldehyde concentrations, emission rates of materials under different ventilation conditions could be simulated by the diffusion mass transfer models if emission parameters are given. The emission parameters depend on the structure of materials, VOC types, as well as environmental conditions. Temperature and humidity, variable in actual buildings, are two key factors influencing formaldehyde emissions once the materials are determined.

Studies had been conducted to identify the effects of temperature and humidity on formaldehyde emissions. Correlations between emission parameters and temperature had been obtained either empirically or theoretically [12], [13], [14], [15]. The studies of humidity effect on formaldehyde emissions, on the other hand, were much less. The studies were typically done experimentally with identical temperature and changed humidity case-by-case. Myers and Nagaoka [16] measured urea-formaldehyde (UF)-bonded particleboard at 25 °C and 40 °C at two different relative humidity (RH) of 30% and 75%. They found that formaldehyde emission increased 2-fold and 6-fold, respectively. Myers [17] reviewed the literature before 1985 together with his own data, and concluded that formaldehyde emission increased at a higher humidity. Frihart et al. [18], [19] measured formaldehyde emission from a UF-bonded particleboard and found that the emission rate increased 6–9 times when RH increased from 30% to 100%. Parthasarathy et al. [20] found that steady-state formaldehyde concentration increased 1.8–3.5 times when RH increased from 50% to 85%. However, these studies mainly focused on analyzing concentrations or emission rates at steady-state or equilibrium conditions. Correlations or quantitative results regarding the humidity effect on the emission parameters are very limited. Moreover, as humidity remained constant during each test in the presented studies, formaldehyde emission behavior under variable humidity conditions is still unknown. Given the variable humidity characteristics in actual buildings, this topic deserves further investigation. Additionally, previous experimental studies regarding the humidity effect were mainly focused on the analysis of RH. Absolute humidity (AH), which represents the absolute quantity of free water content in the air, was seldom analyzed. Measurement results and correlation analysis of a recent field study in a full-scale experimental room suggested that AH was more likely the parameter affecting formaldehyde emission [21]. This however needs to be verified by laboratory data.

The objective of this study is to investigate the humidity effects on formaldehyde emission parameters from a medium-density fiberboard (MDF) and obtain empirical relations. Small-scale environmental chamber tests were conducted under different humidity scenarios. Experiment on the formaldehyde emissions under changing humidity was also performed to roughly approximate the emissions under variable humidity conditions in actual buildings.