Water is often described as tasteless, but anyone who has compared freshly chilled water with water that has been sitting on a table for several hours may notice a difference. The change can be especially noticeable with sachet water, bottled water and water served in a glass.
The important point is that room-temperature water does not automatically become unsafe simply because its taste changes. Several physical, chemical and environmental factors can alter how water tastes and smells over time.
Why does water taste different at room temperature?
One major reason is temperature itself.
Cold water generally tastes fresher and more refreshing because temperature affects how our taste and smell systems perceive substances dissolved in water. As water warms, its sensory characteristics can become more noticeable. Minerals, dissolved gases and very small amounts of other compounds may contribute to the taste that people notice.
There is also a difference between water that has been freshly chilled and water that has been sitting exposed to the surrounding environment.
1. Water interacts with the air
Once water is exposed to air, gases from the atmosphere can dissolve into it. Carbon dioxide is particularly relevant. When carbon dioxide dissolves in water, a small amount forms carbonic acid, which slightly changes the water’s chemistry.
This does not mean that the water has become “spoiled.” Rather, the water’s chemical environment has changed slightly, and some people can perceive the resulting difference in taste.
The effect is generally more noticeable in an open glass or uncovered container because the water is directly exposed to the surrounding air.
Why can sachet water taste different?
Sachet water is different from water that is poured into a glass because the packaging material and storage conditions can influence its sensory characteristics.
Sachets are commonly made from plastic films. While food-contact plastics are designed to be suitable for their intended use, packaging can interact with its contents to a limited degree. Storage conditions, particularly temperature and exposure to sunlight, can influence this process.
Research on packaged water has found that elevated temperatures can increase changes in the chemical and sensory characteristics of water stored in plastic packaging. (Nature)
This is why water that has spent considerable time in a hot environment may taste different from water that has been kept in a cool place.
However, it would be incorrect to conclude that every change in taste means chemicals have entered the water at a dangerous level. Bottled-water regulations require producers to meet safety and quality standards, including requirements concerning chemical and microbiological contaminants. (U.S. Food and Drug Administration)
Heat makes packaging-related changes more important
Temperature is particularly important.
Scientific research examining packaged water at temperatures ranging from about 23°C to 60°C found that higher temperatures were associated with greater changes in water quality and odour, with some chemical compounds increasing during prolonged storage at elevated temperatures. (PubMed)
Another study measured volatile compounds migrating into ozonated water from several plastic packaging components — including bottle caps and liners — with some of these compounds having very low sensory thresholds, meaning they can affect taste or smell even at very small concentrations. (PubMed)
Therefore, a sachet or plastic bottle left in a hot room, near a heat source, in direct sunlight or inside a hot vehicle is more likely to develop an unusual taste than one stored in a cool, shaded location.
Does bottled water also change taste?
Yes, bottled water can also taste different after being left at room temperature.
But there are several possible explanations, and they should not all be attributed to the plastic bottle.
For unopened bottled water, the original water source, mineral composition and treatment process can influence its taste. Bottled water may contain naturally occurring minerals, and different treatment methods can also affect its chemical composition and sensory properties. (U.S. Food and Drug Administration)
The bottle itself can also play a role over sufficiently long periods, particularly when the water is exposed to higher temperatures.
Most single-use water bottles are made from polyethylene terephthalate (PET) or other food-contact plastics. A large-scale study of the Spanish bottled-water market found that plastic components and additives can migrate into water during bottling and storage, with the packaging material and bottle format both affecting how much migration occurs. (Food Chemistry)
That does not mean that drinking water from a normal PET bottle is inherently dangerous. Rather, it demonstrates that packaging and storage conditions can contribute to changes in the water’s characteristics.
What about water in a glass?
A glass of water can also taste different after sitting at room temperature, but the reason is somewhat different.
If you pour water into a clean glass and leave it uncovered, the water is directly exposed to the atmosphere. Over time, gases can exchange between the water and surrounding air. Dust, airborne odours and other environmental factors can also affect the sensory quality of exposed water.
The glass itself is generally much less likely to contribute a noticeable plastic-like taste because glass is relatively inert compared with plastic packaging.
However, glass does not prevent the water from changing at all. An uncovered glass can still develop a different taste because the water is exposed to air and the surrounding environment.
So, if the same water is placed in a clean glass, sealed bottle and sachet and all three are left at room temperature, they may not taste exactly the same later.
Open water versus sealed water
This distinction is important.
| Storage method | Can taste change? | Main factors |
|---|---|---|
| Open glass | Yes | Air exposure, temperature, environmental odours and contamination |
| Sealed glass bottle | Yes, but usually less from the container | Water chemistry, temperature and storage time |
| Sealed plastic bottle | Yes | Temperature, storage time, water chemistry and packaging |
| Sachet water | Yes | Packaging, temperature, storage conditions and water chemistry |
| Open plastic bottle | Yes | Air exposure, temperature, packaging and possible contamination |
The duration also matters. A glass of water sitting on a table for a few hours is not equivalent to bottled water stored for months. Likewise, water kept at a normal indoor temperature is very different from water exposed to high temperatures.
Why does cold water often taste better?
There is also a psychological and sensory component.
Cold water tends to feel more refreshing, particularly when someone is thirsty. Cooling can also make certain flavours and odours less noticeable. As water warms, subtle characteristics that were difficult to detect when it was cold may become more apparent.
Consequently, when someone says, “This water tastes different now that it is warm,” that observation does not necessarily mean that the water has undergone a major chemical change.
Sometimes the temperature has simply changed how the existing characteristics of the water are perceived.
Does room-temperature water become unsafe?
Not necessarily.
A change in taste alone is not a reliable test of whether water is safe to drink.
For commercially packaged water, safety depends on factors such as the quality of the source, treatment, packaging, storage conditions and whether the package remains intact. Regulatory authorities require bottled-water producers to control microbiological and chemical quality during production. (U.S. Food and Drug Administration)
However, once a bottle or sachet has been opened, the situation changes. Drinking directly from a container can introduce microorganisms, and leaving an opened container at room temperature can give microorganisms opportunities to multiply, depending on conditions.
For this reason, opened water should not simply be treated in the same way as an unopened, factory-sealed package.
What if the water tastes strongly of plastic?
A mild difference in taste does not automatically indicate a health hazard. Nevertheless, a strong chemical, plastic-like, solvent-like or otherwise unusual taste or smell is a reason to be cautious.
The safest approach is to avoid drinking water if:
- the package is damaged, leaking or unusually swollen;
- the water has an unusual smell or appearance;
- the package has been exposed to excessive heat for a prolonged period;
- the water has been left open for a long time;
- the container appears dirty or contaminated; or
- you suspect the water has been contaminated.
In particular, packaged drinking water should be stored away from direct sunlight and excessive heat. Studies of bottled water have identified elevated temperature and unsuitable storage as factors that can affect its quality. (Nature)
Why sachet water may seem to change faster
There is no universal rule that sachet water changes taste faster than bottled water. Different products use different packaging materials, thicknesses, manufacturing processes and water-treatment methods.
Nevertheless, sachet water is usually packaged in a relatively thin plastic film. Its storage environment can therefore be important. If sachets are transported or stored in hot conditions, the water can experience substantially higher temperatures than ordinary indoor room temperature.
That is particularly relevant in hot climates, where the temperature of the packaging can become much higher than the surrounding indoor temperature when exposed to direct sunlight.
The important distinction is therefore not simply “sachet versus bottle.” It is:
What material is the water packaged in, how long has it been stored, and under what temperature and environmental conditions?
The bottom line
Water can taste different after being left at room temperature, and this can happen with sachet water, plastic bottles and water in a glass.
The change can result from several factors:
- Temperature changes how we perceive taste and smell.
- Water exposed to air can exchange gases with the atmosphere.
- Minerals and other dissolved substances contribute to water’s natural taste.
- Packaging can influence taste, particularly during prolonged or warm storage.
- Heat can increase interactions between water and some packaging materials.
- Open water can pick up environmental odours or become contaminated.
Therefore, it is not accurate to say that water “goes bad” merely because it has reached room temperature. At the same time, storage conditions matter, especially for packaged water kept for long periods or exposed to heat.
For the best taste and quality, keep packaged water sealed, cool and away from direct sunlight, and once opened, treat it as exposed water rather than as a permanently protected product.
Disclaimer
This article is intended for general educational purposes and is not a substitute for professional advice or laboratory testing. Taste or smell alone cannot establish whether water is microbiologically or chemically safe. If water appears contaminated, the packaging is damaged, or the water has an unusually strong chemical or foul odour, it is safer not to drink it.
References
Diduch, M., Polkowska, Ż., & Namieśnik, J. (2013). Factors affecting the quality of bottled water. Journal of Exposure Science & Environmental Epidemiology, 23, 111–119. Nature journal article
Food and Drug Administration. (2025). Bottled water everywhere: Keeping it safe. U.S. Food and Drug Administration. FDA — Bottled water safety
Food and Drug Administration. (2025). FDA regulates the safety of bottled water beverages including flavored water and nutrient-added water beverages. U.S. Food and Drug Administration. FDA — Bottled water regulations
Greifenstein, M., White, D. W., Stubner, A., Hout, J., & Whelton, A. J. (2013). Impact of temperature and storage duration on the chemical and odor quality of military packaged water in polyethylene terephthalate bottles. Science of the Total Environment, 456–457, 376–383. PubMed — Research on temperature and storage
Guart, A., Bono-Blay, F., Borrell, A., & Lacorte, S. (2014). Effect of bottling and storage on the migration of plastic constituents in Spanish bottled waters. Food Chemistry, 156, 73–80. PubMed — Study on plastic migration in Spanish bottled water
Song, Y. S., Al-Taher, F., & Sadler, G. (2003). Migration of volatile degradation products into ozonated water from plastic packaging materials. Food Additives & Contaminants, 20(10), 985–994. PubMed — Research on packaging and off-tastes
