The relative humidity may strongly affect indoor air concentrations of VOC pollutants

Volatile organic compounds (VOCs) may be emitted from a building into the indoor air leading to a compromised indoor air quality (IAQ). We studied the influence of relative humidity (RH) on VOCs concentrations in a room of a building that had been subjected to water damage. Air samplings at RH 21–22 % revealed only small amounts of 2-ethylhexanol (3 μg/m 3 ) and 2,2,4-trimethyl-1,3-pentanediol diisobutyrate (TXIB, 8 μg/m 3 ) whereas samplings performed at RH 58–75 % gave 3-fold higher concentrations of 2-ethylhexanol and 1.5-fold higher concentrations of TXIB. In a second series of experiments we found that increasing the RH from 40 to 85 % resulted ín dramatic increase in the release of n-butanol and trichloroanisole from a sill that had previously been impregnated with chlorophenols, taken from a building with IAQ complaints. This study illustrates the importance of monitoring RH when sampling indoor air for VOCs to achieve reliable results. Peer-review under the responsibility of the organizing committee of the ICMB23.


Introduction
Buildings may emit a variety of chemicals into the indoor air.Examples are polycyclic aromatic hydrocarbons (PAH, from creosote) and chloroanisols (from chlorophenols used for wood impregnation), mould products including VOCs and toxins, hydrolysis products of plasticizers in plastic mats or glue, a range of unidentified odorous substances etc.Such emissions are largely moisture driven and associated with BRI (building related illnesses) involving respiratory symptoms, skin and mucous membrane irritation, fatigue, reduced productivity at work and school etc. BRI can, however, effectively be avoided by using an emissions barrier to prevent spread of the chemicals mentioned into the indoor air [1].
It is known that the RH may affect the rate of emissions of VOCs from surfaces.In the present study we measured, at different RH, VOCs emissions, both in a building with unsatisfactory IAQ and in a climate chamber containing samples of building materials that had previously been impregnated with chlorophenols.

Materials and Methods
1.A small room (5.7 m 2 ) with a PVC flooring was studied.Air samples had previously showed distinctive amounts of 2ethylhexanol (a hydrolysis product of plasticizers typically used in PVC mats) and TXIB (a plasticizer commonly found in indoor air, sometimes assigned as a semi VOC), strongly indicating moisture driven emissions from the floor.The room was equipped with a humidifier and the ventilation rate was 0.38 air change/h; there was a disturbing odour.Four samples were taken by pumping air through Tenax® TA tubes (30 min, 100 mL/min) followed by analysis wirh gas chromatographymass spectrometry (GC-MS).The first sample was collected before the humidifier was switched on (22 % RH), and two additional samples were taken consecutively when the RH in the room exceeded 58 %.During the third sampling, the humidity peaked at 75 % RH; the humidifier was then shut down automatically to reach 48 % at the end of the sampling (2.5 h after the humidifier had been switched on).The last sampling was performed 16 h later (see Table 1).
2. Five pieces of an impregnated wooden floor sill (approximately 180 g) with an unpleasant mould-like odour were studied.The material had been collected from a moisture damaged building with complaints as regards the IAQ.The samples were transferred to a stainless steel climate chamber (560×480×400 mm inside, 108 l) with two sampling ports.For purification of the incoming air a tube containing Anasorb® 747 was used in one of the ports; chamber air samplings were conducted using a tubing passing through the other port.The ventilation rate was 0.06 air change/h during the experiment.The sill samples were kept in the chamber for 2 h (30 °C, 40 % RH) followed by air sampling.After 24 h, the same samples were again placed in the climate chamber and kept there for 2 h (30 °C, 85 % RH) following air sampling.VOCs were collected by pumping air through Tenax® TA tubes for 30 min at a flow rate of 100 mL/min and then analyzed by GC-MS.

Results
1. Increased air concentrations of 2-ethylhexanol and TXIB were found shortly (approximately 1 h) after increasing the RH in the room.The concentrations decreased again at lower RH 16 h later (Table 1).The room temperatures were 22-24 °C.2. The concentration in the climate chamber of n-butanol rose from 2 μg/m 3 (40 % RH) to 43 μg/m 3 (85 % RH); the corresponding increase for trichloroanisole was from 1 to 10 μg/m 3 .

Discussion
Spread of emissions from a building into the indoor air is often moisture driven and a major cause of BRI.The problem can be prevented by attaching an emissions barrier indoors at the surfaces from where the emissions are spreadon floor, walls or ceiling.We have used the surface emissions trap (cTrap) developed at Lund University Sweden to stop emissions of PAH, chloroanisoles, and phthalate hydrolysis products in PVC mats in buildings with perceived unsatisfactory IAQ [1][2][3] .This is an efficient, economic, quick, and environment friendly way of ensuring a healthy indoor air.However it has not always been possible to detect the pollutants by using standard sampling and analytical methods.The results herein reported illustrate the importance of keeping control of RH when sampling VOC for indoor air analysis: At low RH the air concentrations of the emitted chemicals might be below the detection limit of the analytical methods applied while being readily detecteable at higher RH.Possible explanations for this phenomenon may be that the water vapor evaporation rate is reduced at high RH thus facilitating vaporization of VOCs, and/or competition between VOCs and water molecules for free sorption sites of a studied material.In summary, this study shows that it is vital to keep control of the RH during sampling in order to achieve reliable analytical results as concerns indoor air concentrations of VOC.

Table 2 .
Air concentrations (μg/m 3 ) of butanol and trichloroanisole at different RH