Air Pollution: Is There an Association with Rheumatic Disease?

NO², also known as nitrogen peroxide, is a reddish-brown gas at room temperature and has numerous industrial applications, such as serving as a nitrating agent for explosives and as an oxidizing agent in rocket propulsion.¹⁵ Potential occupation exposures to NO² include fuel combustion in motor vehicles and stationary power plants, etching of metal with nitric acid, as well as acetylene and electric cutting and welding.¹⁵ In agricultural settings, fermentation of recently stored agricultural silage can also produce NO². Motor vehicle traffic is a common source of NO² exposure in the general population, and indoor sources include poorly vented gas ranges, and kerosene or gas space heaters.¹⁵ NO² is readily absorbed and nearly all inhaled compound is absorbed through the tracheo-bronchial tree.^12,15

Less frequently assessed are exposures to black carbon and carbon monoxide (CO), both produced by the incomplete combustion of hydrocarbon materials, such as during the operation of internal combustion engines.^16,17

Ozone is also a pollutant, produced when nitrogen oxides (NOx) and volatile organic compounds (VOCs) interact under the action of sunlight.¹⁸ NOx are also generated by fossil fuel combustion, such as from motor vehicle engines; power plants; industrial plants; and boilers; VOC sources are motor vehicle emissions, gasoline vapors and chemical solvents.¹⁸ Ozone creation, destruction and dispersion are highly dependent on temperature, wind speed and direction, solar radiation and humidity.¹⁸ High concentrations of ozone are observed over large cities and metropolitan areas during warm months.¹⁸

Measuring Air Pollution Exposure in Research Studies

Most metropolitan cities have dedicated automated fixed-site continuous monitoring stations that collect hourly mean levels of criteria air pollutants (i.e., O3, NO², SO², CO, PM10 and PM2.5). The hourly data can be averaged to obtain daily estimates for the entire city and provide air pollution data for temporal analyses; however, these estimates are often not precise across geographic regions.

Air pollution is a recognized risk factor for respiratory & cardiovascular disease, but has also been associated with immune-mediated diseases, such as inflammatory bowel disease & multiple sclerosis.

Land-use regression (LUR) or inverse density weighting is applied to estimate pollution exposure at a given geographical point, typically an individual’s location of residence (determined by postal code). LUR models use Geographic Information System (GIS) to estimate air pollutant exposure through a combination of land use, traffic, population density, physical geography and meteorology across an area and predict concentrations at a defined site.^19,20

Inverse distance weighting uses the general principle of interpolation, where values of a given variable are estimated and derived from a known and measured value at another point (e.g., a monitoring station), and sources farther from the reference point will have a lower impact of the pollution.²¹