Environmental pollution is the introduction of chemical, physical, or biological agents into the environment that modify natural characteristics. These harmful substances, known as pollutants, can contaminate the air, water, or soil. Air quality contamination occurs in both the ambient, open-air environment and within enclosed structures. Understanding the distinct nature of air pollution across these two settings is necessary to grasp the full scope of human exposure.
Defining Outdoor Environmental Contaminants
Outdoor pollution involves the contamination of the open atmosphere across large geographic areas. Primary sources include emissions from mobile sources like cars and trucks, and stationary sources such as power generation facilities and industrial activity. Natural occurrences, including wildfires, dust storms, and volcanic eruptions, also contribute to the overall burden of outdoor air pollution.
One concerning outdoor contaminant is particulate matter (PM), which includes coarse particles (PM 10) and fine particles (PM 2.5). These microscopic solids or liquid droplets are generated by combustion processes and can remain suspended in the air. Ground-level ozone is another common outdoor pollutant, which forms when nitrogen oxides and volatile organic compounds react in the presence of sunlight.
Other significant gaseous pollutants originate mainly from the burning of fossil fuels for energy and transportation. Nitrogen oxides (NOx), for instance, are released from vehicle exhaust and power plants. Sulfur dioxide (SO2) is primarily produced by the combustion of coal and crude oil, particularly from industrial and utility processes. Carbon monoxide (CO) is also a product of incomplete combustion, with major sources being vehicle emissions and fires.
Defining Indoor Environmental Contaminants
Indoor pollution refers to the presence of harmful substances within enclosed structures. The sources of these contaminants are often unique, stemming from a combination of building materials, household activities, and products. Inadequate ventilation is a major factor in the buildup of these pollutants, preventing the dilution of emissions and the removal of contaminated air.
Volatile Organic Compounds (VOCs) are a broad category of indoor contaminants released as gases from various solids or liquids. Common sources include household cleaning products, paints, sealants, new furniture, and certain pressed wood products. Biological agents, such as mold, mildew, pet dander, and dust mites, also contribute to poor indoor air quality, often thriving in areas with excess moisture.
Combustion-related pollutants are introduced inside through appliances and activities like cooking and heating. Nitrogen dioxide and carbon monoxide are byproducts of burning natural gas, kerosene, or wood, and their levels can become hazardous, especially with improper venting. Naturally occurring radioactive gases like radon can also infiltrate a structure from the soil beneath and become trapped indoors, posing a long-term risk.
Comparing Exposure Dynamics
The mechanisms of human exposure to indoor versus outdoor air pollution differ significantly due to the physical characteristics of the environments. People spend a large majority of their time, often cited as 80% to 90%, within enclosed spaces, which increases the duration of exposure to indoor contaminants. Unlike the open atmosphere, the lack of dispersion indoors means that pollutants tend to accumulate.
This accumulation leads to much higher concentrations of certain pollutants inside compared to outside. Indoor air pollutant levels are commonly found to be two to five times higher than their outdoor counterparts, and sometimes they can exceed outdoor levels by over a hundred times. Even energy-efficient building designs, which limit airflow to conserve energy, can inadvertently exacerbate this issue by minimizing air exchange.
The air quality inside is also influenced by the air outside, as ambient pollutants infiltrate buildings through windows, doors, and ventilation systems. However, indoor air quality is often dominated by internal sources, meaning activities occurring within the space, such as cooking or using household products, are the primary determinants of the air breathed inside. This interplay creates a complex exposure scenario where both environments contribute to the overall pollutant burden.
Overall Impact on Human Health
Exposure to polluted air, regardless of its origin, affects multiple physiological systems in the body. The primary pathway is through the respiratory tract, where the inhalation of particles and gases leads to inflammation and oxidative stress. Short-term exposure can cause immediate respiratory symptoms like coughing and wheezing, while long-term exposure contributes to chronic conditions.
The respiratory system is particularly vulnerable, with exposure linked to the development or aggravation of asthma, bronchitis, reduced lung function, and chronic obstructive pulmonary disease (COPD). Fine particulate matter is especially hazardous because its small size allows it to penetrate deep into the lungs and enter the bloodstream.
The cardiovascular system is also profoundly affected by air pollution, which is considered a major risk factor for several diseases. Pollutants can trigger systemic inflammation, leading to conditions such as heart disease, stroke, and irregular heartbeat. Continuous low-level exposure over time contributes to the progression of chronic issues.