Air Pollution Medical Briefing

Smog air pollution

Written by Dr Adrian Hyzler, Chief Medical Officer and Head of Medical Communications

Updated: December 2019

‘Smog’ is a kind of air pollution, named for the mixture of smoke and fog in the air during the late 19th and early 20th centuries. Historically, it was produced by the burning of large amounts of coal within a city. This smog contained soot from smoke, sulphur dioxide and other particulate matter.

Modern smog, for example in Los Angeles, Beijing and New Delhi, is a type of air pollution derived from vehicle emissions from internal combustion engines and from industrial fumes. These particles react in the atmosphere with sunlight. This then produces secondary pollutants that combine with the primary emissions to form photochemical smog, also known as ground level ozone. In contrast, ozone found in the earth’s upper atmosphere, is beneficial and forms a protective layer that shields us from the sun’s harmful UV rays.

Other sources of air pollution include:

  • Coal fires, either domestic or industrial, emitting significant clouds of smoke that contribute to smog – still a problem in fast-developing cities, for example in India, Indonesia and China.
  • Erupting volcanoes can emit high levels of sulphur dioxide along with a large quantity of particulate matter.
  • The radiocarbon content of some plant life, for example the ‘creosote bush’ in LA, has been linked to the distribution of smog in some areas.
  • Deforestation caused by forest fires, used by landowners to clear areas for further planting is a major cause of seasonal air pollution in many South American and South-East Asian cities.
  • Saharan dust has also been carried by prevailing winds over Europe, precipitating a smog-like atmosphere.

Globally, air pollution is increasingly recognised as a significant public health concern, and is likely to be a long-term issue in many of the world’s highly polluted cities. As smog levels increase it becomes highly toxic to humans and can cause severe sickness, breathing problems, shortened life or death. In March 2014, the World Health Organisation (WHO) announced that air pollution kills 3.7 million people worldwide every year. Most of these deaths take place in developing countries, where the problem is at its worst, but hundreds of thousands are also believed to occur in the developed world each year.

AFFECTED AREAS | What areas are most affected by air pollution?

Smog can form in any climate where industries or cities release large amounts of air pollutants. It is worse during periods of warm, sunny weather when the upper air is warm enough to inhibit vertical circulation. It is especially prevalent in geologic basins encircled by hills or mountains.

  • Delhi, India – this is the most polluted city in the world and it is estimated that air pollution causes over 10,000 deaths every year. The problem of high emissions is exacerbated by crop burning in adjacent states. Since the mid-1990s, the government has taken measures to try to address the problem including: mass tree-planting programmes; the introduction of the largest fleet of environmentally friendly compressed natural gas buses; banning the use of leaded petrol and supporting alternative fuel initiatives. However, many of these gains have been lost due to an increase in stubble burning, a rise in market share of diesel cars and decline in bus use.
  • Mexico City, Mexico – the city’s location in a highland ‘bowl’ results in cold air trapping the industrial and vehicle pollution underneath, turning it into the most polluted city in Latin America.
  • Los Angeles and San Joaquin Valley, USA – these areas, notorious for smog because of their locations in low basins that are surrounded by mountains, are subject to the fumes pouring out of millions of vehicles, as well as the emissions from port complexes at San Francisco and Long Beach.
  • Ulaanbaatar, Mongolia – massive immigration in the 1990s to the outskirts of the capital city with the subsequent burning of fossil fuels for heating, has led to severe pollution. The particulate matter levels are among the worst in the world – when inhaled these particles settle in the lungs causing health problems.
  • South East Asia – cities in Indonesia, Malaysia, Singapore and Thailand, are especially affected by land and forest fires, used by landowners to clear areas for further planting. This kind of pollution is usually referred to as ‘haze’ rather than smog.
  • Beijing, China – there is a severe smog problem in major cities in China, especially Beijing, caused by the rapid rate of economic growth resulting in widespread industrial and vehicle pollution. On Feb 25 2014, Beijing had been shrouded in heavy smog and hazardous levels of fine particulate matter (PM2•5) for 6 consecutive days. That morning the PM2•5 level read 383 μg/m3, which is 15 times the recommended safe WHO limit (25 μg/m3), but not the worst reported, in a week when levels soared to over 500 μg/m3.

HEALTH EFFECTS | What effects can air pollution have on your health?

There is mounting evidence that on a population basis, increasing levels of air pollution, particularly PM 2.5 (fine particulate matter <2.5 µm in diameter), are linked to an increased incidence of negative respiratory and cardiac health effects. There is emerging but as yet inconclusive evidence about the effects on premature births, intrauterine growth restriction, and lung development throughout the life course.

Ground level ozone, sulphur dioxide, nitrogen dioxide and carbon monoxide are especially harmful for ‘susceptible’ individuals, such as the elderly, pregnant women, children and people with known heart and lung conditions such as asthma, bronchitis/emphysema, heart failure.

Higher levels of air pollution can cause breathing problems, eye and nose irritation and makes people more susceptible to infections. Hospital admissions and deaths often increase when ozone levels are high. Studies over the last 20 years by the American Cancer Society have found that cumulative exposure increases the likelihood of premature death from respiratory disease. Two recently published European studies found that regularly breathing in air tainted with even low levels of air pollution raises your long-term risk of lung cancer. That finding came from a review of data from nearly 313,000 people across nine European countries. The study was published in The Lancet Oncology.

Studies in areas of California have linked smog with an increased incidence of birth defects affecting the spinal cord. In October 2013, studies of some 74,000 births in 12 European countries concluded that babies in polluted areas were more likely to have low birth weight and a smaller head circumference. A major study across 25 cities has reckoned that living near major urban roads could account for 30 per cent of all asthmas in children.

Short-term exposure to smog has also been linked to an increased risk of hospitalisation or death from heart failure, according to a study led by the University of Edinburgh that reviewed data from 12 countries worldwide, published in The Lancet.

From an organisational point of view, it is important to pre-screen all travellers with pre-existing medical conditions that may be exacerbated by travel to heavily polluted cities. The benefit of this is to either anticipate problems that may be faced and, for example, formulate an action plan, OR alternatively to re-consider whether this is an appropriate destination for the traveller.

PREVENTION | What can I do to prevent exposure to pollutants?

When visiting cities with high levels of air pollution, such as Delhi, Kuala Lumpur, Jakarta, Mexico City etc., the best thing you can do is to reduce your exposure to the pollutants by avoiding main roads and busy streets where possible. If you have a respiratory or cardiac condition this is even more important.

On days when pollution levels are low, you shouldn’t be worried about going outside. However, even on these days, it’s a good idea to avoid spending long periods of time in places where pollution levels build up, such as busy roads – particularly if you have a lung condition.

The effect of ambient air pollution can be either short or long term. In both instances, the cardiovascular and respiratory systems are most likely to be affected. ‘Host factors’ such as age and underlying medical conditions also influence an individual’s susceptibility. ‘Susceptible groups’ include children, pregnant women, the elderly and individuals with known underlying heart or lung disease. These individuals should be more cautious in regards to their exposure to high levels of air pollution.

Health risks associated with high pollution loads are related to both the level of hazard and the period of exposure, so reducing one or both will reduce risk. In the context of air pollution, the actual hazard is beyond the control of the organisation but the level of staff and family exposure, particularly in the workplace and residences, and related to activities can be addressed through a range of organisational control measures and individual education directives.

Many people wonder whether they should wear a face mask…

At the moment there’s very little evidence to recommend the use of ‘simple’ paper/fabric face masks. However, sophisticated masks with active charcoal filters such as N95 respirators, (3M or Vogmasks, for example) can help filter out contaminants including dust, fumes, mists as well as microbial agents- they are certified to filter greater than or equal to 95% of all challenged particles free of oil and greater than 0.3 microns in size- that’s much smaller than PM 2.5 micron particles. N99 respirators filter greater than or equal to 99% etc. It should be noted that many people find wearing a mask very uncomfortable and some people with a lung condition report finding breathing more difficult when there’s something covering their mouth.


As air pollution levels rise, susceptible individuals are at an increased risk of becoming ill and needing treatment. When levels are high, the local government will issue an air pollution alert. It’s sensible to take extra precautions on these high pollution days:

  • Stay informed about the official pollution warning levels, especially ozone levels- see ‘How to Assess the Level of pollution in a City’ below.
  • Reduce or avoid strenuous, outdoor exercise. If you have a lung condition, exercise has many benefits, so if possible, keep doing your exercise indoors in a well-ventilated room or gym.
  • Stay away from pollution hotspots such as main roads and busy road junctions.
  • Try to get to work a little earlier before rush hour has begun and levels of pollution have built up.
  • If you need to use taxis for commuting, make sure the windows are closed and insist that the AC is switched on- use a good quality taxi service.
  • If you cycle, run or walk as part of your commute, use back streets away from the bulk of vehicle congestion.
  • If possible, try to take long weekends in less polluted parts of country (if these exist) for psychological wellbeing.
  • If you have asthma, make sure that you carry your reliever inhaler with you at all times, and use it regularly.
  • People with known respiratory conditions should visit their doctor prior to any planned visit to a polluted city and formulate an “escalation plan”- this may be in the form of an “Asthma Action Plan” – for example, see:
  • Work and living accommodation can be pre-evaluated to ensure that, where there are issues with air pollution, appropriate measures such as draught proofing, air purifiers and air-conditioning units are in place.
  • If you find your medical condition is getting worse, or if you’re getting wheezy or coughing from walking outside, you should call your medical helpline as you may need to see a local doctor. If you’re out and about, you could also call into any pharmacy, where a pharmacist may also be able to give you advice.

Measuring air pollution levels

The US Environmental Protection Agency (EPA) has developed an Air Quality Index in order to provide the public with timely and easy-to-understand information on local air. AQI is usually calculated using the concentration (ug/m3) of PM 2.5. These particles of less than 2.5 microns (µm) in diameter, often referred to as “fine” particles, are now recognised as posing the greatest health risks. US Embassies around the world publish real-time AQI data, free of charge. For example, the New Delhi AQI can be viewed at the following website:

An AQI value of 100 generally corresponds to the national air quality standard for the pollutant, which is the level that has been set to protect public health. AQI values below 100 are generally thought of as satisfactory. When AQI values are above 100, air quality is considered to be unhealthy- at first for certain sensitive groups of people, then for everyone as AQI values get higher (see the table below).


Air pollution level

Health implications

Cautionary statement (for PM2.5)



Air quality is considered satisfactory, and air pollution poses little or no risk.




Air quality is acceptable; however, for some pollutants there may be a moderate health concern for a very small number of people who are unusually sensitive to air pollution.

Active children and adults, and people with respiratory disease, such as asthma, should limit prolonged outdoor exertion.


Unhealthy for sensitive groups

Members of sensitive groups may experience health effects. The general public is not likely to be affected.

Active children and adults, and people with respiratory disease, such as asthma, should limit prolonged outdoor exertion.



Everyone may begin to experience health effects; members of sensitive groups may experience more serious health effects.

Active children and adults, and people with respiratory disease, such as asthma, should avoid prolonged outdoor exertion; everyone else, especially children, should limit prolonged outdoor exertion.


Very unhealthy

Health warnings of emergency conditions. The entire population is more likely to be affected.

Active children and adults, and people with respiratory disease, such as asthma, should avoid all outdoor exertion; everyone else, especially children, should limit outdoor exertion.



Health alert: everyone may experience more serious health effects.

Everyone should avoid all outdoor exertion.


WHO: Review on evidence on health aspects of air pollution, 2013 –
US Environmental Protection Agency –
The Lancet: Effects of Pollution in China’s Major Cities –
Air Now – Air Quality Index –