Marko Hyttinen

Reactions of Ozone on Ventilation Filters

When supply air passes through a filter, ozone and other oxidants may react with the dust and with the compounds adsorbed on it causing decomposition of ozone and producing oxidation products. These reactions have been studied on nine used supply air filters in the laboratory. Some consumption of ozone was observed in almost all the filters. The changes in the concentrations of individual organic compounds due to penetration through the filter were negligible, except for formaldehyde which was produced. Air velocity in the range of 0.1-0.2 ms−1 and ozone concentration from 20 to 60 ppb did not have an effect on reduction. In field experiments, the reduction in ozone concentrations varied from 8 to 26%; the highest reduction was obtained in a HVAC unit with a three-stage-filtration. Sensory evaluation demonstrated that the highest odour emission was caused by the pre-filter. At elevated ozone concentration, the panelists could not detect any odour differences before and after the filters. Expectedly, the reactions did not change the VOC concentrations appreciably, but the reaction products with low odour threshold concentrations may, nevertheless, have some influence on the odour perception.

Adsorption and desorption of selected VOCs in dust collected on air filters

Abstract Adsorption and desorption properties of the dust accumulated on air filters were examined by using a small-scale test apparatus with model compounds. The dust samples were loaded with the model compounds either by adsorption from a constant concentration in air flow or by direct injection into the dust. Desorption was measured at three different relative humidities of air (4–5%, 40–50%, 70–80%). Results indicated that constant relative humidity (RH) of air did not affect the rate of desorption in the test conditions. However, an increase in humidity substantially increased desorption of the model compounds. Similar results were obtained when experiments were conducted using dirty filters without added model compounds. In addition, emission products from clean and dusty filter materials were analyzed at two temperatures (50°C and 100°C) by using an automatic thermodesorption device. The main compounds released were carboxylic acids, aldehydes and terpenes. The emission profiles were similar for the pre-filters and main filters, but the emissions were higher from pre-filters than from the main filters. This result is consistent with the earlier findings of higher odor emissions from pre-filters.

Airborne Infection Isolation Rooms – A Review of Experimental Studies

Ventilation guidelines for airborne infection isolation rooms (AIIRs) are highly variable in different countries indicating lack of actual knowledge about the guidance needed. However, US guidelines for AIIRs are extensive and have been widely adopted outside the US. AIIR performance has also been evaluated in numerous studies. For a long time, the aim has mainly been to evaluate how well the existing AIIRs meet US guidelines. For historical reasons, mixing-type ventilation has been emphasised and attention has been paid to air exchange rates, although the use of auxiliary devices, such as portable room-air cleaners and ultraviolet germicidal irradiation systems, has also been examined. Recently, the scope of the investigations has been widened. The most crucial issue is to minimise the potential for disease transmission and prevent the escape of contaminated air from the AIIR. Airflow direction inside the AIIR is also important and AIIRs minimise air leakage to save energy. On the other hand, it has been observed that efficient containment can be achieved even by using simple and inexpensive construction by considering pressure differential and air flow patterns. Nevertheless, additional research is needed to assist hospitals with improving their preparedness to cope with the threat of pandemics by building and using effective AIIRs.

Performance testing of engineering controls of airborne infection isolation rooms by tracer gas techniques

The ventilation performance of airborne infection isolation rooms (AIIRs) was assessed in three Finnish hospitals by examining the air change rate, contaminant removal efficiency and leakage of contaminants outside the isolation room by using tracer gas techniques. Results showed that infectious agents can escape from the AIIR during egress despite high ventilation rates in the AIIR and anteroom (air change rate, 4–24 h−1) and the pressure difference between the AIIR and corridor was −0.2 to −29 Pa. The control of impurities was often ineffective due to inappropriate direction of air flows and air moving from the patient towards the health care worker and anteroom. Although an anteroom reduces leakage of infectious agents to the corridor significantly, it does not prevent this completely when healthcare workers move between the AIIR and corridor. To enhance the protection of AIIRs, it is especially important to pay attention to the air distribution and removal efficiency of impurities in AIIRs and anterooms. Performance of AIIRs should be tested regularly, especially among older AIIRs.