Environmental health is a massive pillar of Preventive and Social Medicine (PSM), and it's basically the "bread and butter" for exams like NEET-PG and INI-CET. Let's break down these three big topics—water, air, and the hazards of the workplace—into high-yield chunks.

First, I'll pull up some key lessons so you have the full details ready to go.

1. Water Purification

Water purification is usually divided into large-scale (for cities) and small-scale (for households or emergencies).

Large-Scale Purification: The most high-yield comparison you'll need to know is between Slow Sand Filters (Biological Filters) and Rapid Sand Filters (Mechanical Filters).

• Slow Sand Filters: These rely on the Vital Layer (Schmutzdecke), which is a zoogleal layer that does the actual filtering. They are great for removing bacteria but take up a lot of space.
• Rapid Sand Filters: These use chemical coagulation (like Alum) and are much faster, though they require more skilled operation.

Chlorination: This is the most common method for disinfecting water.

• ==Free Residual Chlorine==: There should be at least 0.5 mg/L of free residual chlorine in the water after 30 minutes of contact time.
• Horrocks' Apparatus: Used to estimate the dose of bleaching powder needed to disinfect a given amount of water.

I've got a great comparison table for the sand filters that makes this much easier to memorize.