When you need to fill an air-conditioning circuit, protect a solar thermal installation against frost or cool a process in the food industry, the same question keeps coming up: ethylene glycol, propylene glycol or monoethylene glycol? All three names appear in technical data sheets, quotations and catalogues, and it is not always clear whether they are different products, which one gives better freeze protection or which one is safer.
The short answer is that monoethylene glycol and ethylene glycol are the same compound, and that the differences that matter are between ethylene glycol and propylene glycol. At the same concentration, both offer very similar antifreeze protection. What sets them apart is their toxicity, their viscosity (and with it the thermal performance of the circuit) and the applications in which each one is permitted or recommended.
In this article we compare their properties using data, explain what concentration you need according to the minimum temperature of your installation and help you choose the right glycol for each case, drawing on the more than 30 years of experience we have built up at CH Química manufacturing and supplying industrial glycols and antifreezes.
Ethylene glycol and monoethylene glycol: are they the same?
Yes. Monoethylene glycol (MEG) is the technical and commercial name for ethylene glycol (1,2-ethanediol, C₂H₆O₂, CAS 107-21-1). The prefix “mono” is used in industry to distinguish it from other glycols in the same family, which are obtained in the same manufacturing process from ethylene oxide:
- Monoethylene glycol (MEG): a single ethylene glycol unit. It is the main product and the one used as an antifreeze and heat transfer fluid.
- Diethylene glycol (DEG): two units joined together. It is used as a solvent, a humectant or in natural gas dehydration, but it does not replace MEG in thermal circuits.
- Triethylene glycol (TEG): three units. Its main use is gas drying, along with some solvent applications.
Therefore, if one supplier offers monoethylene glycol and another offers ethylene glycol, they are talking about the same substance. What can vary is the purity grade: fibre-grade MEG, with purities above 99.9%, is used to make polyester and PET, while industrial grade is used in antifreezes and thermal fluids. And when people in air conditioning or automotive simply talk about “glycol”, they almost always mean ethylene glycol.
Something similar happens with propylene glycol: it is also sold as monopropylene glycol (MPG) or 1,2-propanediol, and all these names refer to the same product. If you want to learn more about it, we explain what propylene glycol is and what it is used for in industry in another post on this blog.
Ethylene glycol vs propylene glycol: comparison of properties
Both are diols, that is, alcohols with two hydroxyl groups. They are colourless, practically odourless, hygroscopic liquids that are miscible with water in any proportion. They are very similar, but their differences have direct consequences for the design and safety of an installation.
| Property | Ethylene glycol (MEG) | Propylene glycol (MPG) |
|---|---|---|
| Chemical formula | C₂H₆O₂ | C₃H₈O₂ |
| CAS number | 107-21-1 | 57-55-6 |
| Molecular mass | 62.07 g/mol | 76.09 g/mol |
| Density (20 °C) | ≈1.113 g/cm³ | ≈1.036 g/cm³ |
| Boiling point | ≈197 °C | ≈188 °C |
| Freezing point of the pure product | ≈−13 °C | ≈−59 °C (vitrifies, does not crystallise) |
| Dynamic viscosity (20 °C) | ≈20 mPa·s | ≈55 mPa·s |
| Specific heat (20 °C) | ≈2.4 kJ/(kg·K) | ≈2.5 kJ/(kg·K) |
| CLP classification | Harmful if swallowed (H302). May cause damage to the kidneys through prolonged or repeated exposure by ingestion (H373) | Not classified as hazardous |
| Food use | Not permitted | Authorised as additive E1520 (food grade or USP) |
| Relative cost | Cheaper | Slightly more expensive |
Three differences that decide the choice emerge from the table: toxicity, viscosity and low-temperature behaviour. Let us look at them one by one.
Toxicity: the difference that weighs the most
Ethylene glycol is toxic if swallowed. The body converts it into glycolic acid and oxalic acid, which cause metabolic acidosis and kidney damage. The toxicological literature puts the potentially lethal dose for an adult at around 1–1.5 ml per kilo of body weight, that is, in the order of 100 ml. It also has a sweet taste that increases the risk of accidental ingestion by children and pets, which is why many ethylene glycol-based antifreezes include bittering agents.
Propylene glycol, on the other hand, is metabolised into lactic acid and pyruvic acid, compounds that the body processes normally. The US FDA recognises it as GRAS (Generally Recognized As Safe) and in the European Union it is authorised as a food additive under code E1520. This does not make it harmless at any dose, as we discuss in Is propylene glycol toxic or safe to handle?, but its safety margin is far greater than that of ethylene glycol.
The practical consequence is clear: in any installation where a leak could come into contact with food, drinks, drinking water or people, the glycol must be propylene glycol, and in food applications, food-grade or USP grade.
Thermal performance: why ethylene glycol is more efficient
At the same concentration, ethylene glycol has a noticeably lower viscosity than propylene glycol, and the difference grows as the temperature drops. A more viscous mixture has three effects on the circuit:
- It needs more pumping power to move the same flow rate.
- It reduces the heat transfer coefficient in heat exchangers and coils, because the flow is less turbulent.
- It may force you to oversize pumps or heat exchangers in circuits that operate at very low temperatures.
That is why, in closed industrial air-conditioning or refrigeration circuits with no risk of contact with food or drinking water, ethylene glycol is usually the most efficient and economical option. When safety requires propylene glycol, this must be taken into account in the design: the hydraulic and thermal calculation must be made with the real properties of the mixture, not with those of water or of an ethylene glycol mixture.
It is also worth remembering that any glycol reduces the ability of water to carry heat. That is why it makes no sense to use a higher concentration than the installation needs.
Antifreeze protection: what concentration you need
Something that surprises many installers: at the same percentage by weight, ethylene glycol and propylene glycol protect against freezing in a very similar way. The difference between them is not in how much cold they withstand, but in their toxicity and their viscosity.
| Glycol concentration (% by weight) | Freezing point with ethylene glycol | Freezing point with propylene glycol |
|---|---|---|
| 10 % | ≈−3 °C | ≈−3 °C |
| 20 % | ≈−8 °C | ≈−7 °C |
| 30 % | ≈−14 °C | ≈−13 °C |
| 40 % | ≈−22 °C | ≈−21 °C |
| 50 % | ≈−34 °C | ≈−33 °C |
Four rules for getting the concentration right
- Calculate based on the real minimum temperature of the installation, with a margin. A common practice is to size the protection about 5 °C below the expected minimum.
- More concentration is not more protection. Maximum protection is reached at around 60%, close to −50 °C; above that, the freezing point stops dropping and even rises again (pure ethylene glycol freezes at about −13 °C). In addition, every extra point reduces heat transport capacity and increases viscosity.
- Do not fall short either. Below 20–25% the protection is minimal, the inhibitors become too diluted and the risk of bacterial growth in the circuit increases, especially with propylene glycol.
- Check the units. Manufacturers’ tables may be expressed as a percentage by weight or by volume, and the two are not equivalent. Before dosing, confirm which unit the table you are using is in.
We review the most common dosing and maintenance failures in our guide to the correct use of industrial glycols in thermal systems.
Pure glycol or inhibited antifreeze
Pure glycol, whether monoethylene glycol or propylene glycol, lowers the freezing point, but it does not protect the circuit against corrosion. With temperature and oxygen, glycol oxidises and generates organic acids (glycolic, formic, acetic) that lower the pH and attack metals such as steel, cast iron, copper or aluminium.
That is why, in closed heating, cooling or solar energy circuits, the recommended option is an antifreeze formulated with corrosion inhibitors, keeping pure glycol for those who formulate their own fluids or for processes in which the inhibitor is added separately. We explain how these products work in the article on industrial antifreezes to protect circuits and equipment.
Which one to choose? A guide by application
Given all of the above, the choice depends mainly on two questions: can a leak reach food, drinking water or people? And what thermal performance does the circuit need? This table summarises the most common applications:
| Application | Recommended glycol | Why | CH Química product |
|---|---|---|---|
| Closed industrial heating and cooling circuits, with no risk of contact with food or drinking water | Inhibited ethylene glycol | Better heat transfer and lower cost | AF GLYCOL HRE -20 (ready to use) or AF GLYCOL HRE -60 (concentrate) |
| Heating and air conditioning in buildings or installations where a low-toxicity fluid is preferred | Inhibited propylene glycol | Lower risk in the event of a leak | AF GLYCOL HRP -60 |
| Solar thermal energy | Propylene glycol formulated for solar circuits | Low toxicity in installations that share a heat exchanger with domestic hot water | AF GLYCOL SOLARTECH -40 |
| Food industry: cold rooms, wineries, breweries, dairies | Food-grade propylene glycol | A leak does not pose a toxicological risk to the product | AF GLYCOL FOOD GRADE -60 |
| Food, pharmaceutical or cosmetic formulation | USP propylene glycol | Minimum purity of 99.5% and documented traceability | PROPILENGLICOL USP |
| Technical formulations with propylene glycol with no food requirement | Technical propylene glycol | Low toxicity at a lower cost than USP grade | PROPILENGLICOL |
| Manufacture of own-brand antifreezes, chemical processes, polyester | Pure monoethylene glycol | Versatile raw material for formulation | MONOETILENGLICOL |
The whole range is available in our Glycol Line of glycols and antifreezes for water circuits.
Maintenance: how to monitor the glycol in the circuit
Choosing the right glycol is half the job. The other half is keeping it in good condition throughout the life of the installation:
- Measure the concentration at least once a year with a refractometer with a scale specific to each glycol: a reading from a refractometer calibrated for ethylene glycol is not valid for propylene glycol, and vice versa.
- Monitor the pH and the inhibitor reserve. A pH that drops over time indicates that the glycol is degrading and that the inhibitors are being depleted.
- Do not mix ethylene glycol and propylene glycol. They are miscible, but the mixture makes it impossible to measure the concentration reliably and combines inhibitor packages that may not be compatible. If you change glycol, drain and clean the circuit first.
- Top up with the same mixture, not with water alone: every top-up with water dilutes the protection.
- Dilute with demineralised or low-hardness water to avoid scaling and the precipitation of the inhibitors.
- Avoid galvanised steel in circuits with glycol: zinc reacts with the inhibitors and with the glycol degradation products.
- Clean the circuit before charging. Sludge, magnetite and residues of previous fluids degrade the new glycol. Equipment such as Clearflow circuit cleaning pumps makes it possible to flush the installation and then refill it with glycol.
Frequently asked questions about ethylene glycol and propylene glycol
Which is better, ethylene glycol or propylene glycol?
It depends on the application. Ethylene glycol transfers heat better and is cheaper, so it is usually preferred in closed industrial circuits with no risk of contact with food or drinking water. Propylene glycol is the only option when a leak could affect food, drinks, drinking water or people, because of its low toxicity.
Is monoethylene glycol toxic?
Yes. Monoethylene glycol is the same compound as ethylene glycol and shares its toxicity: it is harmful if swallowed and can cause kidney damage. It must be handled with gloves and goggles, stored properly labelled and out of the reach of children and animals, and always used in accordance with its safety data sheet.
Can ethylene glycol and propylene glycol be mixed?
Chemically they are miscible, but mixing them in a circuit is not recommended: the real concentration can no longer be measured reliably and the inhibitors in each formulation may not be compatible with each other. If you switch from one glycol to the other, the right approach is to drain, clean and refill the installation.
Which glycol is used in solar thermal installations?
Normally propylene glycol, formulated specifically for solar circuits. Its low toxicity is important because the primary circuit exchanges heat with the domestic hot water, and the formulation must withstand the high temperatures that collectors reach in summer.
Which glycol does car antifreeze contain?
Most automotive antifreezes and coolants are based on ethylene glycol (monoethylene glycol) with inhibitor additives. Propylene glycol-based formulations, which are less toxic, also exist, although they are less common.
How often should the glycol in a circuit be changed?
There is no fixed interval: it depends on the type of fluid, the operating temperatures and the condition of the circuit. The recommended approach is to analyse the fluid at least once a year (concentration, pH and condition of the inhibitors) and replace it when the values fall outside the range indicated by the manufacturer.
Choose the right glycol with technical advice
Ethylene glycol and monoethylene glycol are the same product: efficient and economical, but toxic. Propylene glycol protects practically the same against frost, with far lower toxicity, in exchange for higher viscosity. Choosing between them is not a question of quality, but of matching the fluid to the installation, the regulations and the risk of each application.
At CH Química we help you choose the right glycol and concentration for your circuit, with the technical data sheet and the safety data sheet of each product. Contact our technical team and tell us which installation you need to protect.