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London smog, also known as classical smog, was a prevalent issue in cities like London during the early to mid-20th century. It emerged primarily from the burning of coal combined with high humidity levels, creating a dense mixture of smoke and fog. This type of smog is termed 'reducing' because it contains chemicals that can react to remove oxygen from their environment. The smog would often appear as a thick, dark haze, severely affecting air quality. People exposed to this kind of air pollution often experienced respiratory issues and other health problems.

Unlike photochemical smog, which is oxidizing, London smog is reducing. This is a crucial difference and demonstrates why regulating coal burning and improving air quality standards has been critical in preventing this type of pollution.

Photochemical smog is a type of air pollution commonly observed in urban environments, especially those with plenty of sunlight and vehicle emissions. This smog is formed through complex reactions involving nitrogen oxides, volatile organic compounds, and sunlight, creating harmful substances like ozone and peroxyacetyl nitrate (PAN).

Photochemical smog is termed 'oxidizing' due to the presence of strong oxidants such as ozone that can react aggressively with other compounds. Unlike the reducing conditions in London smog, photochemical smog actively oxidizes pollutants in the atmosphere. These reactions can lead to various adverse health impacts, notably respiratory problems and eye irritation, due to its composition.

Peroxyacetyl nitrate, commonly abbreviated as PAN, is a significant component of photochemical smog. It forms as a secondary pollutant, meaning it is not directly emitted. Instead, it results from chemical reactions in the atmosphere involving primary pollutants like nitrogen oxides and volatile organic compounds.

PAN is particularly concerning due to its extended stability in the atmosphere, allowing it to transport pollution over long distances. It acts as an eye and respiratory irritant, contributing to the discomfort associated with smog exposure. Understanding PAN's role in air pollution helps highlight the importance of controlling vehicle emissions and industrial pollutants that contribute to its formation.

Eye irritation is a common symptom experienced by people exposed to photochemical smog. This irritation is primarily caused by oxidative pollutants such as ozone and PAN. These chemicals can damage the delicate tissues in the eyes, leading to discomfort and, in severe cases, even exacerbating conditions like asthma and bronchitis.

Eye irritation serves as an indicator of significant air quality issues and reinforces the need for effective pollution control measures. Simple actions like reducing vehicle emissions and industrial activities can significantly impact reducing the occurrence and intensity of photochemical smog and its associated health effects.

Understanding the difference between oxidizing and reducing conditions helps explain the characteristics and effects of different types of smog. Oxidizing conditions, found in photochemical smog, involve substances that add oxygen or remove hydrogen from other compounds. These conditions are aggressive, leading to the formation of harmful pollutants like ozone and PAN.

In contrast, reducing conditions, as seen with London smog, involve elements or compounds that can donate electrons, typically removing oxygen from the atmosphere. These smogs have different chemical profiles and health impacts, highlighting the importance of distinguishing between them for effective pollution regulation and management. This differentiation aids in crafting targeted strategies to mitigate their harmful effects on the environment and public health.