Just like clouds of fog that form in humid conditions because of the temperature difference between the air and a surface; eye-glasses fog-up due to temperature differences between the lens and surrounding air. This can happen when you move from a cold environment to a warmer one or the other way around. This is particularly common in oil and gas, power, warehousing and other industries where workers regularly move from indoor to outdoor environments or vice versa.
It can also occur as a reaction to the warmth emitted by your face in contrast to the cooler surrounding air. Essentially, moisture in the air condenses on the surface of the glass in response to the temperature difference. This is of particular concern in agriculture, construction and other settings where manual labourers exert effort and heat up as they work.

A number of factors can increase the chances of this fogging effect. Dirty and damaged lenses, for example, create more surface area upon which condensation can form. This dirt increases the fogging effect and is particularly pronounced in older glasses and goggles. In order to reduce accidents at work, businesses must ensure that their employees are equipped with new and undamaged eyewear but also that lenses are cleaned frequently.

Man on a construction site wearing safety goggles

High humidity environments, both indoor and outdoor, are likely to result in foggy lenses regardless of temperature differences. However, longer exposure to cold temperatures can cause lenses to chill completely. Meaning any warmth will result in prolonged fogging despite wiping and other efforts. Warm factories and plants in cold environments are the perfect examples, as workers move from outdoors into the warmth they are at high risk of fogging-up. Many workers in these environments are left in a ‘Catch 22’ situation. Wearing foggy eyewear reduces their visibility preventing them from working productively and increasing the risk of accidents. Many of those workers chose to remove their protective eyewear in order to see better, which in turn puts their eyes at risk from other dangers such as light, heat and airborne particles. 90% of all workplace eye injuries can be avoided by using proper safety eyewear, according to the Prevent Blindness Organisation, but foggy eyewear creates another set of safety risks. Nearly 60% of workers sustaining eye injuries were not wearing eye protection at the time, according to The Bureau of Labor Statistics. This may be linked to the goggles fogging up, insufficient personal protective equipment (PPE) training or other reasons.

Man wearing safety goggles and mask.

Where fogging up can cause serious hazards, such as workplaces that operate heavy
machinery, it is essential to not only have anti-fog but also anti-scratch protection on lenses. Anti-fog coating incorporates hydrophilic materials that absorb moisture and hydrophobic techniques that divert excess moisture to the sides of the lens. Most protective eyewear on the market only protects the inside of the lens against fogging and the outside against scratching but this does not account for all the types of fogging up that can occur. Other technology like Platinum Coating by Bollé Safety offer more comprehensive protection from fogging up by applying scratch-and-fog-resistant coating on both the inside and outside of the lens.