Sulphur Dioxide and Asthma

Air quality is determined by measuring five major pollutants:1

  1. ground level ozone
  2. particulate matter (PM)
  3. carbon monoxide (CO)
  4. sulfur dioxide (SO2)
  5. nitrogen dioxide (NO2)

and the result, called the “Air Quality Index”, or AQI,  is a number between 0 to 500. The AQI works in reverse – the higher the number, the poorer the air quality. An AQI below 100 is considered acceptable.
The EPA has assigned specific colours to each category of air quality.

It should be noted that any person may experience the health effects of pollution within a few hours or even days after breathing polluted air.

In the past, research has paid less attention to SO2 than to the other components of AQI. Recent publications indicate that SO2 may have a greater impact than previously considered. Sulfur dioxide is a toxic, water-soluble, colourless gas. It is a reductant that causes irritation of the nose, eyes and respiratory tract, asthma symptoms, and decreased lung function. Prolonged exposure can result in asthma-like symptoms in individuals without asthma and can aggravate existing cardiovascular disease.

SO2 emissions are produced from industrial processes, though metal extraction from ores and by power plants and refineries. It is also produced when fossil fuel is burned. It has a rotten-egg odour which can be noted at levels between 0.3 and 5 parts per million (ppm). At 5 to 10 ppm it causes coughing and a burning sensation in the eyes. The EPA has set a standard for exposure in one hour of 75 parts per billion (ppb) or 0.75 ppm. However, exposure to levels as low as 0.1 to 0.5 ppm can result in bronchoconstriction in persons with asthma.2

Exposure to SO2 causes3, 4

  • airway inflammation
  • increased airflow resistance
  • pulmonary dysfunction
  • increased epithelial permiability
  • oxidative stress
  • amplification of allergic inflammation
  • neurogenic inflammation
  • immune dysregulation

SO2 is inhaled through the nose which provides a sufficiently moist environment to convert the gas into both sulfurous and sulfuric acid. While these acids can be absorbed into the lining of the airways they can also react with other airborne chemicals to bond to particulate matter of different sizes and thereby be carried into the airways and into the alveoli. These acids also irritate the airways and set up an inflammatory response.

Research has shown that the inflammatory response triggered is magnified in individuals with asthma due to their lack of sufficient interleukin - 10 (IL-10) needed to counter this type of inflammation and that these individuals have more bronchoconstriction.1,2 A study of children under the age of 6 noted a correlation between the average monthly concentration of SO2 and the use of prescribed inhaled bronchodilators.5 See Table 1. Yet another study found that for every one degree Celsius reduction in ambient temperature and a 1 ppb increase in SO2 concentrations, the risk of acute asthma exacerbations within 1 to 2 days6 increased by 14.8% and 19.7 % respectively among non-smokers.

Both short- and long-term effects of SO2 on children have been studied. Short-term effects include an increase in respiratory symptoms, reduced lung function, more emergency department visits and hospital admissions for asthma and other respiratory reasons. Long-term effects include airway remodeling, increased prevalence of respiratory symptoms and respiratory disease. A study in Montreal of children who lived near a petroleum refinery suggested a connection between SO2 emissions and the “prevalence of active and poor asthma control”.7

Sulfur dioxide is toxic and a pollutant. Individuals with asthma are more sensitive to this gas and have a greater inflammatory response because of their inability to produce IL-10. The adverse effects are seen in asthmatics even with very small amounts of this gas is present.

(For more information on the effects of the components of air pollution please read the article titled “A breath of fresh air . . . and what else?” The article is sub-titled “Air Pollution and Asthma”.)


  1. Accessed July 2015.
  2. Reno AL, Brooks EG, Ameredes BT. Mechanisms of heightened airway sensitivity and responses to inhaled SO2 in asthmatics. Environ Health Insights. 2015 Apr 1;9(Suppl 1):13-25. doi: 10.4137/EHI.S15671.
  3. Johns DO, Linn WS. A review of controlled human SO(2) exposure studies contributing to the US EPA integrated science assessment for sulfur oxides. Inhal Toxicol. 2011 Jan;23(1):33-43. doi: 10.3109/08958378.2010.539290.
  4. Guarnieri M, Balmes JR. Outdoor air pollution and asthma. Lancet. 2014 May 3; 383(9928): 1581–1592. doi:  10.1016/S0140-6736(14)60617-6
  5. Durmiševi S, Durmiševi -Serdarevi J et al. The correlation of average monthly ambient sulfur dioxide concentration and the use of inhalation bronchodilators in children. Med Glas (Zenica). 2012 Aug;9(2):397-400. Abstract.
  6. Kim S, Kim Y, Lee M et al. Winter season temperature drops and sulfur dioxide levels affect on exacerbation of refractory asthma in South Korea: a time-trend controlled case-crossover study using soonchunhyang asthma cohort data. J Asthma. 2012 Sep;49(7):679-87. doi: 10.3109/02770903.2012.702839. Abstract.
  7. Deger L, Plante C, Jacques L et al. Active and uncontrolled asthma among children exposed to air stack emissions of sulphur dioxide from petroleum refineries in Montreal, Quebec: a cross-sectional study. Can Respir J. 2012 Mar-Apr;19(2):97-102.