What’s in the air I breathe?
Thursday, 11 August 2011 at 2:27:PM
Getting a handle on the amounts of volatile organic compounds present in the air
Speaking as an analytical chemist, I quite happily use ‘parts per billion volume’ (ppbv) to describe concentrations of volatile organic compounds (VOCs) in the ambient atmosphere – but it’s easy to lose sight of what these values actually represent in real life.
The air we breathe is made up predominantly of permanent gases, along with a variable amount of water vapour. The VOCs too vary substantially from place to place, but the most concentrated is methane (1.8 ppmv), with all the other organic compounds being in the low ppbv range. To get an idea of just how little these amounts are, see the accompanying graphic.
Outdoor air is one thing, but indoor air is quite another. With emissions of VOCs from indoor furnishings an area of concern at the moment, it’s worth taking a closer look at these. A pretty well-known one is styrene (the monomer that goes to make polystyrene). Widely acknowledged to be a ubiquitous component of indoor air, it’s released from a variety of materials used in interior furnishings, and is the subject of lots of regulations due to its suspected health effects. But how much might you actually breathe in over the course of a day?
Let’s say you’re in a new office, and it’s a cold day, so the heating’s on and the windows are closed, with not much fresh air circulating. A browse around the web suggests that the indoor styrene concentration might be 2 ppbv, so let’s start with that. Given that you breathe in 10 litres of air per minute when at rest, that’s 4200 litres in a 7-hour working day. Multiplying that by 2 × 10–9 gives 8.4 µL of pure styrene vapour every day, which is equivalent to 0.34 µmol, or 36 µg (using the the molar volume of an ideal gas of 24.4 L to get a rough idea). It’s not very much really – less than the weight of a grain of table salt – and certainly much less than you get in the lab if you open a solvent bottle outside the fumehood!
OK, so that was a relatively high-concentration component – what about something at the opposite end of the spectrum? CFC-113 (or Freon® 113) first appeared in the atmosphere in 1961, and underwent a sharp rise in concentration following widespread use as a refrigerant and solvent. However, following the Montreal Protocol, its concentration levelled off in the late 1990s, at about 84 ppt. Because its principal decomposition route is by the action of light in the stratosphere, its concentration is falling at only 1 ppt a year. Incidentally, the worldwide concentration is pretty uniform too (now that production is essentially zero, it’s had time to ‘equilibrate’), making it a useful ‘internal standard’ for air measurements.
Working through the maths again, and using its current concentration of 74 ppt, we find that, at today’s concentrations, you would inhale just 23 mg of CFC-113 over the course of an 80-year lifetime – about the mass of a grain of rice. Using another analogy, 74 ppt equates to just 0.2 mL out of an Olympic-sized swimming pool (2.5 million litres). And yet modern thermal desorption technologies wouldn’t have much trouble in detecting such low levels. That’s pretty amazing really.
However, low as those levels are, Nature’s inevitably got one up on us – at least as far as 1-p-menth-1-ene-8-thiol is concerned. This C10 compound (a monoterpene thiol) is responsible for the fresh juicy smell of grapefruit, and can be detected by the human nose at levels as low as 0.1 ppt. Now there’s a challenge for the gas chromatographer!
Matt Bates, Markes' Thermal Desorption Product Manager
It’s actually pretty difficult to find a single source that gives authoritative concentrations for all the components of air down to the ppb level, so I’ve had to cull these numbers from a variety of sources, including:
Many components vary substantially depending on the location and the time of year, so the values in the graphic are necessarily a bit rough-and-ready!
See http://www.epa.gov/ttnatw01/hlthef/styrene.html for details of styrene concentrations.
A good website to get the latest on freon concentrations is http://cdiac.ornl.gov/oceans/new_atmCFC.html
For commentary on the grapefruit smell, see: I. Flament and R. Näf, Surfing on the scent waves in the food flavor sea, in Flavor chemistry: 30 years of progress, ed. R. Teranishi, E. L. Wick and I. Hornstein, Kluwer Academic/Plenum Publishers, New York, 1999.
The original grapefruit paper is: E. Demole, P. Enggist and G. Ohloff, 1-p-Menthene-8-thiol: A powerful flavor impact constituent of grapefruit juice (Citrus parodisi MACFAYDEN), Helv. Chim. Acta, 1982, 65(6), 1785–1794.