1. The most appropriate English term for haze is "smog," which originated from the combination of "smoke" and "fog." This term gained popularity during the Industrial Revolution when urbanization led to heavily polluted cities with smoke-enshrouded landscapes.

2. The word "smog" reflects the essence of haze, as it signifies the combination of smoke, emitted from factories, and fog. It is a portmanteau that encapsulates the idea of air pollution, particularly in urban areas.

3. PM2.5 refers to particulate matter with a diameter of 2.5 micrometers or smaller, which can enter the lungs. This measurement is used to quantify the tiny particles that contribute to respiratory issues and other health problems during periods of heavy air pollution.

4. In English, to describe severe air pollution, adjectives like "severe," "serious," or "heavy" are commonly used. For instance:

- "The smog is severe!"

- "When the pollution gets really serious, I can't even see the buildings next to me."

- "Beijing partially bans traffic on heavy pollution days."

These statements express the impact of pollution and the measures taken in response to it.