Urban Indoor Air: Where Pollutants Thrive

Indoor air pollution is a serious global issue that affects human health, causing millions of deaths each year. While outdoor air quality is often associated with pollution, the air we breathe inside buildings can also be polluted. In fact, indoor concentrations of some pollutants are often 2 to 5 times higher than typical outdoor concentrations. This is due to a variety of factors, including inadequate ventilation, human activities such as cooking and smoking, and the use of certain chemicals in household products. Certain populations may be more vulnerable to the effects of indoor air pollution, including children, older adults, individuals with pre-existing conditions, and low-income households. Therefore, it is important to identify the main sources and concentrations of indoor air pollutants and devise strategies to improve indoor air quality.

Cooking and smoking

Cooking and the use of solid fuels for heating are major sources of indoor air pollution. Around 3.8 million people die each year due to exposure to household air pollution, according to the World Health Organization (WHO). In developing countries, solid fuels such as wood, coal, and dung are often burned indoors for cooking and heating. This is also the case in rural areas of high-income countries, such as the US, where 6.5 million people rely on solid fuels for heating. The combustion of these fuels produces gases such as nitrogen oxide, carbon monoxide, and particulate matter, including PM2.5 (tiny particles that are hazardous to human health when inhaled).

Indoor air pollution from cooking stoves alone is responsible for approximately 3.1 million premature deaths each year, primarily affecting women and children, according to the United Nations. Women and children are disproportionately affected by indoor air pollution as they typically spend more time exposed to harmful smoke from polluting stoves and fuels, and they are also often responsible for collecting firewood, which can put them at risk of injury and violence in less secure environments.

The use of modern stoves can also pose a risk of indoor air pollution. For example, a study found that cooking an omelette in a kitchen with a modern stove resulted in higher exposure to PM2.5 than standing on an average London roadside. Frying and roasting are particular sources of indoor air pollution. In addition, the use of fuel-burning appliances, such as cooking stoves, furnaces, and water heaters, can create emissions that are harmful to health and the environment.

To address the issue of indoor air pollution from cooking, the WHO has issued guidelines for indoor air quality and household fuel combustion, which include recommendations on the types of fuels and technologies that protect health. The WHO defines clean fuels and technologies as solar, electricity, biogas, liquefied petroleum gas (LPG), natural gas, alcohol fuels, and biomass stoves that meet emission targets. The use of extractor fans when cooking and HEPA filters in vacuums, air purifiers, and heating and air conditioning systems can also help to reduce indoor air pollution.

Smoking is another major source of indoor air pollution. Tobacco smoke causes ten times more air pollution than diesel car exhaust, making it one of the most severe indoor air pollution sources. Secondhand smoke has been linked to increased respiratory morbidity in individuals with COPD, even in communities that do not use solid fuels for heating or cooking. In addition, indoor air pollution exposure during pregnancy has been associated with impaired lung function in infants. Radon, a naturally occurring radioactive gas that can enter buildings through the ground or cracks in walls, is the leading cause of lung cancer among non-smokers in the US.

Poor ventilation

Infiltration, natural ventilation, and mechanical ventilation are the three main ways outdoor air enters and exits a building. Infiltration occurs when outdoor air flows into a building through openings, joints, and cracks in walls, floors, and ceilings, as well as around windows and doors. Natural ventilation occurs when air moves through opened windows and doors due to air temperature differences and wind. Mechanical ventilation devices, such as outdoor-vented fans, intermittently remove air from a single room.

Poorly designed, maintained, or operated mechanical ventilation systems can contribute to indoor air problems. For example, in an effort to save energy, ventilation systems may not bring in enough outdoor air, leading to increased pollutant levels. This is particularly relevant in office buildings, where mechanical heating, cooling, and ventilation systems are common. Additionally, physical defects in buildings, such as those found in some inner-city schools, can contribute to poor ventilation and, consequently, poor air quality.

In low- and middle-income countries, poor ventilation is a significant issue, especially around biomass-stove use. The use of open fires and inefficient stoves, along with the burning of solid fuels, results in increased indoor air pollution. This is also true for some households in the United States, where 6.5 million people live in homes heated primarily by wood or coal. In these cases, inadequate ventilation can lead to extremely high levels of indoor smoke and fine particles, which are harmful to respiratory health.

The effects of poor ventilation and indoor air pollution can be severe and include respiratory diseases, heart disease, cognitive deficits, and cancer. Certain populations are more vulnerable to the impacts of indoor air pollution, including children, older adults, individuals with pre-existing conditions, and people of low socioeconomic status. Therefore, it is essential to address poor ventilation and improve indoor air quality to protect public health.

Building materials

People tend to spend approximately 90% of their time indoors, and indoor air is more contaminated than outdoor air. Therefore, indoor air quality (IAQ) is an important consideration for human health and productivity.

The use of synthetic building materials and energy-efficient construction methods without sufficient mechanical ventilation can increase indoor pollutant levels. Inadequate ventilation can cause indoor pollutant levels to rise, as it does not bring in enough outdoor air to dilute emissions and carry indoor air pollutants out.

To improve indoor air quality, it is important to promote the use of sustainable, non-toxic, and air-purifying building materials. Additionally, appropriate ventilation strategies, such as natural or passive ventilation, energy-utilising air filtration, or mechanical ventilation, can help to maintain a healthy indoor environment.

The World Green Building Council recommends improving building quality to achieve thermal comfort and reduce the demand for heating and cooling. Buildings designed for their specific climate, using renewable energy sources for heating, lighting, and power, are optimal for reducing localised and large-scale air pollution.

Outdoor pollutants entering

Outdoor pollutants can enter indoor spaces through a variety of mechanisms, and this can have a significant impact on indoor air quality. The outdoor environment is an important factor in determining indoor air quality, and the penetration of outdoor pollutants into indoor spaces is a well-studied phenomenon.

There are three main ways in which outdoor pollutants enter indoor spaces: mechanical ventilation, natural ventilation, and infiltration. Mechanical ventilation is driven by ventilation fans, air conditioners, or central air conditioning systems, which draw in outdoor air. Natural ventilation occurs when doors and windows are open, and air movement is caused by temperature differences and wind. Infiltration is when outdoor air enters through cracks and leaks in the building. The rate of air exchange between the indoor and outdoor environments is an important factor in determining indoor air pollutant concentrations.

Outdoor air pollutants, such as NOx, SO2, O3, CO, HC, and particulate matter (PM), can penetrate indoors and accumulate or become diluted depending on the ventilation conditions. In areas with poor outdoor air quality, such as urban environments or areas with contaminated soil or water, the impact of outdoor pollutants on indoor air quality can be more severe. Energy-efficient buildings with insufficient mechanical ventilation and the increased use of synthetic building materials and household products can also contribute to higher indoor pollutant concentrations.

Additionally, certain human activities can introduce outdoor pollutants into indoor spaces. For example, cooking with solid fuels, such as wood or charcoal, can release pollutants both indoors and outdoors. Smoking, burning solid fuels, and using cleaning products or pesticides can also release pollutants that accumulate indoors. These activities can have a significant impact on indoor air quality, especially in poorly ventilated spaces.

The impact of outdoor pollutants entering indoor spaces can vary depending on geographical location and socioeconomic status. For example, in low- and middle-income countries, a higher proportion of the population relies on polluting fuels and technologies for cooking, contributing to household air pollution. Certain populations, including children, older adults, individuals with pre-existing health conditions, and those of low socioeconomic status, may be more vulnerable to the effects of indoor air pollution.

Cleaning products

Indoor air pollution is a serious issue, causing approximately 3 million premature deaths annually, according to the World Health Organization. While it is a particular problem in developing countries, where indoor burning of solid fuels is common, it is also a concern in developed nations due to the use of cleaning products, paints, and other household items that release pollutants.

To reduce exposure to harmful chemicals from cleaning products, it is advisable to opt for green or natural cleaning alternatives. These alternatives include common household items like white vinegar, baking soda, borax, citrus fruits, and essential oils. These natural ingredients are safe for indoor use and effectively clean surfaces while reducing the release of toxic chemicals into the air.

Additionally, proper ventilation plays a crucial role in maintaining good indoor air quality. Ensuring adequate air circulation helps dilute emissions from indoor sources and carries pollutants out of the indoor environment. Mechanical ventilation devices, such as outdoor-vented fans and air handling systems, can be particularly effective in removing polluted indoor air and replacing it with fresh, clean air from outside.

Furthermore, maintaining a clean and dry indoor space is essential to prevent the growth of mould and mildew, which can be common in areas like the kitchen and bathroom. Promptly addressing any leaks or water damage can help inhibit the growth of these organisms, improving indoor air quality and reducing potential health risks.

Frequently asked questions