8 Sensor Technology MONITORING INDOOR AIR FOR VOLATILE ORGANIC COMPOUNDS
Clean air contains very few volatile organic compounds (VOCs). Methane is only present at 2000 ppb (parts per billion), ethane at 1 ppb and formaldehyde at 0.5 ppb. Higher concentrations of VOCs arise from human activity, burning vegetation, or occasionally volcanic activity.
T
otal emissions of particulates and volatile organic compounds other than methane (NMVOCs) in the UK have decreased by more than 50% since 1990. This is good news, and largely thanks to a dramatic decrease in vehicle emissions, a trend seen in many other countries. Simultaneously, research has revealed in more detail the exposure of people to particulates and VOCs, and their effect on health. Defi nitive thresholds have been compiled for VOC concentrations indoors such as those provided by Public Health England in 2019, presented in the table below.
Compared to the 8 hr time weighted average (TWA) safety thresholds presented in the table, the short term guideline exposure limits are orders of magnitude lower. There are several reasons for this. The guideline limits are advisory and refer to persistent exposure, whereas safety thresholds are mandatory and relate to workplace exposure. The guidelines arise from long-term exposure studies. The tabled data is refl ective of very contemporary long-term studies. Recommended exposure limits have tended to decrease in line with better management of volatiles, more detailed research and societal expectations.
Some public locations present special and exceptional levels of specifi c VOCs. For example, indoor environments located close to transportation, such as petrol stations, bus stations, garages, or airport departure gate entrances, may on occasion be subject to high fugitive fuel emissions. Dry-cleaning shops are exposed to dry- cleaning fl uids such as trichloroethylene. Refectory and restaurant kitchens are liable to generate acetaldehyde or acetamide. In any space where air internally circulated, any solvents in use will accumulate. Organic acid and sulphur containing VOCs are also common ‘nuisance odours’ affecting indoor air quality, though usually the ‘nuisance’ is at concentrations that are not readily discernible with commercial detectors.
Monitoring VOCs indoors
Given the range of VOC’s in air and their variable harmfulness, it might seem preferable to monitor the more ubiquitous and harmful of them individually. Despite the technological revolution of recent decades, analysis of individual VOCs continues to rely
Table. IAQ guideline data from the UK in 2019. The exposure limits have been converted to ppb at 1 bar and 25 o
C. Short term limits are for exposures averages
over 0 to 60 min, long term for a day or a year. ‘EH40 TWA limits’ are provided for comparison, also from the UK. ‘TVOC’ refers to ‘Total Volatile Organic Carbon’. The ppb value is derived from 0.3 mg/m3
assuming a mean molecular weight of 80 g/mol, sourced from European guidelines for building regulations.
upon gas chromatographic separation of volatiles, followed by their detection. The cost and service burden of such VOC analysers remains high – much too high for wide deployment. But some specifi c VOC detectors and sensors are commercially available.
Formaldehyde is a major air contaminant indoors, especially in newly furbished rooms, typically outgassing from fi breboard and plastics or a few months after installation. Fairly inexpensive monitors engaging metal oxide semiconductivity based sensors provide robust responsivity. A potential drawback is their signifi cant cross sensitivity and slow clear down on exposure to other gases. Sensors which provide selective formaldehyde detection at low levels appear to be commercially elusive.
Benzene arises primarily from external sources but is also found in paints. Selective monitors are available which engage miniaturised GC-PID (gas chromatography-photoionisation detection).
Specifi c air quality monitors are unknown for tri and
IET MARCH / APRIL 2023
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60
