Propylene glycol and ethylene glycol look almost the same: clear, slightly sweet, syrupy liquids. Both are used as antifreeze and solvents. But they are not interchangeable. One is safe enough for food and medicines; the other is poisonous. Confusing the two has caused deaths worldwide, including in India, so buyers need to be clear about which one they are ordering.
At a glance
| Propylene glycol (PG) | Mono ethylene glycol (MEG) | |
|---|---|---|
| Chemical name | 1,2-propanediol | 1,2-ethanediol |
| CAS number | 57-55-6 | 107-21-1 |
| Toxicity | Low; permitted in food (E1520) and medicines in the right grade | Toxic if swallowed; damages kidneys |
| Main uses | Food, pharma, cosmetics, food-safe heat transfer, flavours, tobacco and e-liquids | Polyester (PET, fibre) production, engine coolant, industrial heat transfer, gas dehydration |
| Freezing protection | Good | Slightly better per kilo |
| Price | Higher | Lower |
When to choose propylene glycol
Choose propylene glycol whenever people could swallow, inhale or absorb the product, or it could touch food. That covers syrups and oral liquids, creams and lotions, flavours, food colours, bakery and dairy uses, and chilled-water systems in food and pharma plants where a leak could reach the product. Use the USP/IP grade for medicines and the food grade for food, with a COA for every batch.
When ethylene glycol is the right choice
MEG is the standard industrial glycol. It is the raw material for polyester fibre and PET bottle resin, the base of most vehicle coolants, and the cheaper choice for industrial chillers and heat-transfer loops where there is no food or drug contact. It also goes into resins, de-icing fluids and some paints. Handle it as a toxic chemical: label it clearly and keep it far away from any food or pharma store.
The DEG/EG problem
Several cough-syrup tragedies happened because glycerine or propylene glycol was replaced with, or contaminated by, cheaper industrial glycols: diethylene glycol (DEG) or ethylene glycol (EG). Indian regulators now require pharma manufacturers to test each batch of such excipients for DEG and EG. If you make medicines, buy only pharmacopoeial grades from traceable sources, and test them on receipt. See IP, BP and USP grades explained.
Storage
Both glycols absorb moisture from the air and are best kept in closed HDPE or stainless containers. Both are combustible but not highly flammable. Store food and pharma grades separately from industrial chemicals, with clear labels.
Frequently asked questions
Is propylene glycol the same as ethylene glycol?
No. They are different chemicals (CAS 57-55-6 and 107-21-1). Propylene glycol has low toxicity and is used in food and medicines in the right grade. Ethylene glycol is toxic and is for industrial uses only.
Can I use MEG instead of propylene glycol in a food plant chiller?
Not where a leak could reach food or drinks. Food and pharma plants usually use propylene glycol in such systems. MEG suits closed industrial systems with no product contact.
Why must glycerine and propylene glycol be tested for DEG and EG?
Contamination with diethylene glycol or ethylene glycol has caused fatal poisonings from medicines. Indian regulators require pharma makers to test these excipients for DEG and EG before use.
This article is general guidance. Always follow the Safety Data Sheet and your own site procedures for the specific product and grade you use.