Airborne Pollutants: Eroding Our Buildings And Our Health

The deterioration of buildings is caused by a combination of physical, chemical, and human factors. While some buildings are affected by natural weathering, pollutants in the atmosphere can also cause erosion. For example, in coastal environments, there are naturally higher levels of salt in the atmosphere, but human activities such as salting during cold spells can create artificially elevated levels of salt inland. This can cause damage to buildings when the salt migrates to the surface and crystallizes within the pores of the masonry, causing the stone to crumble. Similarly, when organic chemicals from trees and vegetation mix with air pollutants, the resulting corrosive gas can increase the erosion of building materials, including stone, concrete, and steel.

Organic chemicals from vegetation and air pollutants can create corrosive gas, eroding stone, concrete and steel

The presence of vegetation in urban spaces is a double-edged sword. On the one hand, plants can reduce temperatures in cities, cut down exposure to pollution, and provide mental health benefits. On the other hand, organic chemicals released by trees and vegetation, when mixed with air pollutants, can create corrosive gases that increase the erosion of building materials such as stone, concrete, and steel.

This phenomenon has been observed in cities across the globe, from London, where noxious air was causing the degradation of buildings as early as the 1660s, to modern-day China, where a study of 336 cities found that paved surfaces and heat-retaining buildings contributed to a loss in bird diversity.

The damage to buildings is primarily caused by ground-level ozone, which forms when volatile organic compounds emitted by plants are broken down by sunlight and react with nitrogen dioxide, a common pollutant. This process results in the creation of corrosive gases that can erode stone, concrete, and steel.

Heritage buildings constructed from limestone and sandstone are particularly vulnerable to this type of erosion. To mitigate the impact on these structures, local authorities have been encouraged to invest in green spaces and carefully consider the species of vegetation planted in urban areas.

Additionally, the impact of atmospheric pollution on buildings is not limited to the effects of organic chemicals from vegetation. Changes in the natural atmosphere, such as elevated levels of salt in inland locations due to human activities, can also accelerate weathering and cause degradation.

Salt crystallisation, often caused by air pollutants, can cause stone to crumble and break down

Salt crystallisation is a significant factor in the erosion of buildings, and it is often accelerated by air pollutants. Salt damage, also known as weathering, occurs when salt crystallises inside the pores of porous building materials such as limestone, sandstone, concrete, and brick. The crystallisation process generates substantial force, causing the material to break or crumble. This phenomenon has impacted numerous historic structures, including the Egyptian pyramids and marble Grecian monuments.

Salt crystallisation within constrained spaces, such as the pores of building materials, can exert considerable pressure, resulting in the breakdown of the material. This breakdown can manifest in various forms, including flaking, blisters, and honeycomb weathering. The presence of blisters on a surface, for instance, weakens the structure and enhances salt migration within the blister, perpetuating a cycle of deterioration.

The process of salt crystallisation and its impact on buildings are influenced by environmental conditions, particularly relative humidity and temperature variations. High winds, constant heat, and sunlight can contribute to a continuous heating and liquifying cycle, affecting the degree of salt damage. Additionally, the vulnerability of a building to salt damage depends on its history, environment, and the characteristics of the building materials, such as porosity and permeability.

Salts that contribute to crystallisation and subsequent erosion can originate from various sources, including atmospheric pollution, groundwater, seawater, rainwater, rock salts, de-icing salts, and building materials themselves. De-icing salts, for example, have been identified as a significant source of salt damage to adjacent stone structures. When used on drives and sidewalks during winter, these salts introduce additional salts into the environment, which can be transported by wind and water into the walls of nearby buildings.

While green spaces in urban areas offer numerous benefits, research suggests that they may also contribute to the erosion of building materials. Organic chemicals released by trees and vegetation can mix with air pollutants, forming a corrosive gas that accelerates the erosion of stone, concrete, and steel. This effect has been observed to impact heritage buildings constructed from limestone and sandstone. Therefore, while green spaces are essential, careful consideration should be given to the species of vegetation planted to mitigate potential damage to nearby structures.

Chloride ions from water leaking into concrete can cause pitting corrosion

Chloride-induced corrosion is a significant issue for concrete structures, particularly in coastal areas or environments with high levels of chloride. This type of corrosion occurs when chloride ions, often from sources like de-icing salts, seawater, or polluted environments, penetrate the concrete and reach the steel reinforcement within.

Concrete is naturally alkaline, and this alkalinity helps to protect any steel reinforcement by promoting the formation of a passive oxide layer that isolates it from the surrounding environment. However, when chloride ions come into contact with the steel surface, they combine with hydrogen ions in the pore water to create acids. These acids neutralise the alkalinity of the concrete, facilitating the breakdown of the protective passive layer.

The concrete cover above the reinforcement bars is crucial in preventing chloride ion infiltration. Unfortunately, the formation of fissures or cracks in this cover provides a pathway for the ingress of water and chloride ions, aggravating the chemical degradation of the concrete and accelerating steel corrosion. This process is known as pitting corrosion, a localised form of damage where small pits or cavities form on the steel surface due to concentrated chloride ions in specific areas.

Pitting corrosion is particularly dangerous because it can lead to rapid deterioration of the steel reinforcement while leaving the rest of the structure seemingly unaffected. The pits act as stress concentrators, increasing the likelihood of structural failure. Furthermore, pitting is challenging to detect in its early stages, often progressing unnoticed until significant damage has occurred.

Water leakage from nearby sources is one of the primary ways chloride ions enter concrete structures. This can be particularly problematic in underground structures like parking garages or basements, where high humidity levels combine with external chloride sources to initiate corrosion. In addition to water leakage, other factors contributing to chloride-induced corrosion include the use of de-icing agents, exposure to seawater, industrial pollutants containing chloride compounds, and inadequate sealing of concrete surfaces.

Construction and renovation can expose occupants to harmful chemicals and respiratory irritants, causing health issues

The application of tile adhesive, roofing materials, paints, and other products used during renovations can also be sources of volatile organic compounds (VOCs) and other irritating chemicals. For instance, pressed wood products, such as particleboard and plywood panelling, commonly contain formaldehyde, a pungent-smelling gas that can cause watery eyes, burning sensations in the eyes and throat, nausea, and difficulty breathing. At elevated levels, formaldehyde may even trigger asthma attacks and has been shown to cause cancer in animals and potentially in humans.

In addition to these indoor sources of pollutants, outdoor construction-generated pollutants can infiltrate occupied buildings. For example, contractors often use fossil-fuelled construction and heating equipment in indoor areas, and the resulting combustion products, such as carbon monoxide, can migrate into occupied areas. To prevent exposure, it is recommended to seal exterior doors with weather stripping and door sweeps and to cover dirt or debris piles with tarps or wet them down to reduce the aerosolization of particulates.

The impact of these construction and renovation-generated pollutants on occupied areas can be evaluated through air monitoring for VOCs, airborne particles, and products of combustion. Preventing or minimising exposure to these pollutants is essential to reduce indoor air-related health issues in building occupants. For example, the State Department of Education (DOE) in the US amended its regulations in 1999 to require that state-funded construction projects follow guidelines to prevent the exposure of building occupants to construction/renovation pollutants. Similarly, renovations of occupied buildings should be separated from occupied areas by constructing temporary physical barriers, such as those made of plywood and polyethylene plastic.

Lack of routine cleaning and maintenance can cause deterioration of buildings

A building is an asset and can be one of the largest investments anyone can make. Therefore, it is in the owner's best interest to perform proper maintenance or hire professionals to do so. This will help preserve the structural integrity of the building as well as its initial appearance and functionality, thus retaining its value.

Buildings do deteriorate over time due to various factors, and a lack of routine cleaning and maintenance is a significant contributor to this process. Firstly, the external walls of buildings are often exposed to the elements, such as rain and frost, which can cause damage if not properly maintained. Routine cleaning and maintenance are essential to prevent the accumulation of dirt, debris, and pollutants that can accelerate deterioration.

Additionally, moisture penetration through external walls or ground floors can create suitable conditions for the growth of fungi, which can be detrimental to foundations and construction materials. Routine cleaning and maintenance can help identify and address these issues early on. For example, keeping building materials in a dry state can reduce the likelihood of fungi growth.

Furthermore, a lack of routine cleaning and maintenance can lead to poor workmanship, such as using substandard materials or failing to consider the contraction and expansion of materials, resulting in faulty craftsmanship. This can include issues such as improper spacing or ignoring the causes of deterioration, like humidity and fire.

To ensure proper maintenance, building owners should implement a maintenance policy with a scheduled routine and standard procedures for each component. This can include daily, weekly, monthly, yearly, and every five-year routines, with architects and surveyors checking for defects and necessary replacements. By adhering to a maintenance schedule, owners can preserve the value and safety of their buildings while also protecting their investments.

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