This element of a reliability requirement answers the questions of where and under what conditions the product should operate.
It includes storage, transportation, and installation conditions too. One way to think of the environment is to consider the weather around the device. Temperature, humidity, preoccupation, etc.
Stresses acting on the product
Another element of the environment includes stresses acting on the product.
This may include vibration, force, dust, corrosive gases or liquids, lubricants, solar radiation, voltage, current, and many others. The full set of environmental conditions for any product is quite large. The focus should be on the few environmental conditions which adversely impact the product performance over time.
Imagine the product during transport to or from the point of sale. Detail the set of environmental conditions that the product may encounter. The same for storage and installation. The three different phases of the product life may encounter very different sets of conditions.
This also applies to products that during use may experience very different conditions. An aircraft is a classic example as parked at the gate, rolling on the runway, takeoff, flight, and landing each may experience very different conditions and stresses.
For products that operate with supplied power, consider the impact of voltage spikes, sags, drops, and other power line anomalies.
Few products can withstand a direct lightning strike, yet a nearby strike will cause a large voltage spike that may encounter your product. If purchasing a power supply to include in your product, does your supplier fully understand the set of power line conditions where you expect to deploy your product?
Nature of the use
Another example is the nature of the use by the set of possible customers. Some may use the product every day and others only once a month. The set of conditions professional cyclists may expose a bicycle to include a wider range of weather conditions and much higher stresses than a recreational cyclist on the same equipment.
What leads to failure?
To narrow down the set of environmental conditions to a practical list, start with the environmental stresses that lead to product failure. Consider each possible condition and how it may lead to failure then prioritize to the shorter list that will lead to failures over the duration of expected use.
Some conditions will cause immediate failure, such as dropping a typical cell phone into the ocean. Even retrieved the water and salt will quickly cause electronics to corrode and fail. Likewise dropping a cell phone from a moving vehicle onto the pavement is likely to scatter the device. In these cases consider how often the product will experience these destructive conditions and is it worth design a product robust enough to survive.
Some conditions will cause accumulated damage. Thermal cycling causes metal fatigue in solder joints and may cause embrittlement or crack propagation along adhesive joints. The freeze/thaw cycling of water within a crack in granite will fracture boulders. Dust and fibers will accumulate on heat sinks or filters reducing the efficiency of product cooling.
Range of Conditions
Consider the range and behavior of environmental conditions to identify the conditions most likely to cause failures. If not well understood, experiment to determine the possible range of failure mechanisms involved.
When collecting the environmental information attempt to collect sufficient information to determine the population distribution. For example, applied loads on a bicycle sprocket may vary over a range from barely enough force to cause motion, to very high loads of a professional sprinting uphill. The distribution may be described with a nonparametric histogram or a suitable statistical distribution.
Weather data is readily available from the US National Oceanic and Atmospheric Agency (NOAA) website.
By collecting a sufficient range of data, generally over a few years, you can determine the distributions of temperatures, daily temperature changes, precipitation, and wind loads in specific regions of the world.
Ideally, you have information on where the product will be placed in use thus matching the use location, proportion of customers and environmental conditions.