Yesterday I logged into a webinar run by the instrument manufacturer, TSI, on “Exposure to Ultrafine Particles- Indoor, Outdoor and In-Vehicle Concentrations: Sources, and Particle Dynamics”, presented by Dr Lance Wallace a leading researcher in the measurement of ultrafine particles.
The potential risks to health from sub-micron ultrafine particles has been a high profile occupational health issue for a number of years due to the increasing use of “engineered” nanoparticles (over 1000 consumer products already contain them). The risks aren’t properly understood and their measurement presents a number of difficulties.
The normal method for assessing the risk from airborne particulates involves determining their mass concentration. However, with nanoparticles it’s their surface area of nanoparticles which affects their toxicological properties so a different approach is needed. Another problem is that sub-micron ultra-fine particles are generated by natural and other sources, so results are confused by the background concentration of nanoparticles from natural and other sources making interpretation difficult.
The webinar concentrated on environmental concentrations, rather than occupational exposures, and was based on studies carried out in the USA by the Environmental Protection Agency (EPA), the National Institute of Standards and Testing (NIST) and Stanford University, and a major study undertaken by Health Canada.
Major outdoor sources of ultrafine particles are traffic emissions and atmospheric processes leading to “nucleation bursts” (atmospheric reactions involving sulphuric acid, ammonia, and water vapour).
Indoor sources in domestic premises included
- gas heaters
- electric cookers
- electric motors (including power tools and kitchen appliances)
Of these, the main contributor to indoor concentrations (other than smoking) was cooking.
Source: Wallace and Ott (2011)
I was surprised that electric cookers generated ultrafines. This is due to metal vapourising from the cooking element and then condensing. Ultrafines are generated by electric motors by vapourisation of carbon and metal from the brushes. Given the wide use of electric motors in industry this is likely to be a significant background source in the workplace.
Particles from outdoors also infiltrate into buildings, the proportion entering depends on a number of factors including particle size, building design, air change rate and occupant behaviour (e.g. opening windows and doors).
90% of the particles detected in the studies were smaller than 10 nm. The particle size distribution changing over time due to settlement and coagulation (small particles “clumping” together).
Results from studies reported in Wallace and Ott (2011) indicated that the median background (indoor) number concentration was 2,700 particles/cm3 (5th percentile 900 particles/cm3, 95th percentile 9000 particles/cm3).
One interesting fact that came out of the talk was that relatively high numbers of ultrafine particles can be found in restaurants (see chart below). These are generated due to cooking in the kitchen. The measurements were taken in the dining rooms so the levels in the kitchens, to which the restaurant staff would be exposed, would probably be higher. Also, diners would only be exposed to a couple of hours on occasions whereas restaurant employees would be exposed for considerably longer periods on a regular basis.
Particle concentrations measured in a restaurant. Source: Wallace and Ott (2011)
So what are the implications from an occupational hygiene perspective? The key points for me are
- Although the studies discussed during the webinar were concerned with domestic exposures, many of the sources, including gas burners, electric heaters and electric motors, are used in industry and so would also emit ultrafines that would contribute to the background levels in the workplace environment
- Infiltration of ultrafine particles generated by outdoor sources will also occur with industrial buildings
- Workers in restaurants are likely to be exposed to ultrafine particles generated in kitchens. It’s difficult to say whether the risks are significant and, perhaps, it is something worth investigating – if funding was available.
Lance Wallace and Wayne Ott Personal exposure to ultrafine particles Journal of Exposure Science and Environmental Epidemiology (2011) 21, 20–30; published online 20 January 2010
J Grigg The health effects of fossil fuel derived particles Arch Dis Child 2002;86:79-83