µg/m³:  Micrograms (µg) per cubic meter (m³).  Airborne silica levels are usually described in µg/m³ – the amount of silica dusts in each cubic meter of air.  A typical worker breathes about 10 cubic meters of air a day.

U-Factor:  The factor representing resistance to heat flow of various building materials.

UG/M3:   Micrograms per cubic meter.

UL: Underwriters’ Laboratory.

ULC: Underwriters Laboratories of Canada.

Ulocladium: A rapidly growing, dark brown or black fungus that is a potential allergen. Its spores are unusually large (1015 um). It is typically found in soil and is frequently encountered at low levels indoors.
 
Ulp Filter: Ultra Low Penetration Air Filter designed to be 99.997% efficient on 0.3 micron particles when clean.

ULPA:  Ultra Low Penetration Air See: Filter, Ultra Low Penetration Air.

ULPA Filter:  An Ultra Low Penetration Air filter. A HEPA-type filter with higher efficiency filter medium. Originally developed for cleanroom applications in semiconductor manufacture.

Ultrafiltration (UF):  A separation method operating at 50-200 psi in crossflow filtration mode. Efficiency is approximately 90%. Used to separate large molecules according to their molecular weight.

Ultrafine Particles:  Particles in the size range from approximately 0.02µm to the upper limit of detectability of the discrete particle counter.

Ultra-Low Emission Vehicle (ULEV): Vehicles that meet the ARB ultra-low emission standards. These emission limits are between those for LEVs and ZEVs.

Ultra Quiet Operation:  The Siesta is very quiet and can be used in places where silence is important such as bedrooms, offices and meeting rooms.  In LL mode the Siesta7FS will run at 16db.  Dead of night is measured at 20db.

Ultrasonic:  Sound waves that are too high for humans to hear are “ultrasonic.” The hearing range of many animals, including rodents, extends much higher than for humans.

Ultrasonic (processes):  Process that utilizes specially-designed tooling usually vibrating at 15-80 KHz. Processes are designed to cause localized heating of thermoplastic materials which, in turn, will provide some type of welded or fused joint. Benefits are elimination of fillers and minimized heat stress on surrounding materials.

Ultrasonic Humidifier:  A type of humidifier that creates a cool mist through the use of ultrasonic vibrations. Thought to produce dispersions of both microorganisms and minerals.

Ultrasonic Leak Detector:  An instrument designed to detect the ultrasonic emissions and convert them to audible signal.

Ultraviolet: The invisible rays of the spectrum of light which are at its violet end. Sometimes abbreviated U.V.

Ultraviolet Degradation: A reduction in certain performance limits caused by exposure to ultraviolet light.

Ultraviolet Light:  This is a type of light that starts at the upper end of the visible light spectrum and ends at X-rays. There is a wide range of commercial uses for Ultraviolet (UV) Light but in the air and water purification industry it is widely used as a disinfectant, being an effective virucide and bactericide.

Ultraviolet Radiation:  Solar radiation having wavelengths just shorter than visible light.

UM: Micrometer.

Unbalance: The condition of a rotor in which its rotation results in centrifugal force being applied to the rotor’s supporting bearings.

UNC:  Thread – Unified national coarse.

Unconditioned Space:  Area within the outermost shell of a house that is not heated or cooled-the area outside of the thermal envelope. Such areas typically include crawlspaces, attics, and garages.

Undercoat: A coating applied prior to the finishing or top coats of a paint job. It may be the first of two of three coats. In some usage of the word it may become synonymous with priming coat.

Undercounter: A style of lavatory which is positioned under the cutout of the countertop.

Underfloor Air Distribution:  A system using an underfloor plenum (open space between the structural concrete slab and the underside of a raised floor system) to deliver conditioned air directly into the occupied zone of the building. Air is delivered through supply outlets typically located at floor level or integrated as part of the office furniture and partitions. Return grilles are located above the occupied zone. This upward convection of warm air is used to efficiently remove heat loads and contaminants from the space.

Underground Plumbing: The plumbing drain and waste lines that are installed beneath a basement floor.

Underground Injection:  The placing of fluids below the surface of the ground through wells. Fluids injected include hazardous wastes, brine from oil and gas recovery, liquids used in mining, radioactive waste, sewage, natural gas and oil products and storm water runoff. USEPA and states regulate underground injection under the Safe Drinking Water Act and RCRA.

Underground Storage Tank (UST): Refers to tanks used to store gasoline underground.

UNF:  Thread – Unified national fine.

Underlayment: A material placed under finish coverings, such as flooring, or shingles, to provide a smooth, even surface for applying the finish.

Underwriters Laboratories, Inc. (UL):  The leading third party product safety organization in the United States, the largest in North America and the leading quality system registrar headquartered in the United States. Providing product safety verification services for more than a century, the UL Mark is one of the world’s most familiar safety certification symbols. The Canadian Standards Association (CSA) provides similar service in Canada. Recently UL and CSA have been working cooperatively and have adopted joint procedures, standards and marks.

Unducted: System consisting of one or more air cleaners positioned in the overhead space to create a planned air circulation pattern. This method effectively cleans the overall ambient plant air. Unducted systems rely upon constant cleaning and mixing of the entire room volume to remove airborne contaminants. Unducted systems never remove 100% of the contaminants. The objective of this method is to achieve a substantial reduction in steady-state contaminant concentration.

Unicell: Combination of an ionizer and collection cell in a single assembly in a SMOG-HOG electrostatic precipitator.

Unidirectional Airflow:   Air that flows in a single pass, in a single direction, with uniform velocity through a cleanroom or clean zone with generally parallel streamlines. Formerly referred to as Laminar Flow.

Uniform Airflow:  Unidirectional airflow pattern in which the point-to-point readings are within plus or minus 20% of the average airflow velocity for the total area of the laminar flow work zone.

Uniform Flow: Airflow in which velocities between any two given points remain fairly constant.

Uniformity Coefficient:  Separation factor applied to the sizing of the sand used in water filtration plants.

Uniformity of Feed:  Uniformity of the mixture of the solids in the feed liquid.

Union: A plumbing fitting that joins pipes end-to-end so they can be dismantled.

Union Nut: A fitting that joins two sections of pipe.

Unit Risk Number: The number of potential excess cancer cases from a lifetime exposure to one microgram per cubic meter (µ/m3) of a given substance. For example, a unit risk value of 5.5x10-6 would indicate an estimated 5.5 cancer cases per million people exposed to an average concentration of 1 µ/m3 of a specific carcinogen for 70 years.

Unit Type Compressors:  Compressors of 20 HP or less, generally combined with all the components required to put the into operation.

Unit Ventilator:  HVAC term for a fan-coil unit package device for applications in which the use of outdoor – and return – air mixing is intended to satisfy tempering requirements and ventilation needs.

Unitary System:  A room unit which performs part or all of the air conditioning functions. It may or may not be used with a central fan system.

United Nations Framework Convention on Climate Change (UNFCCC):  A treaty signed at the 1992 Earth Summit in Rio de Janeiro that calls for the “stabilization of greenhouse gas concentrations in the atmosphere at a level that would prevent dangerous anthropogenic interference with the climate system.”  The treaty includes a non-binding call for developed countries to return their emissions to 1990 levels by the year 2000.  The treaty took effect in March 1994 upon ratification by more than 50 countries. 

United States Environmental Protection Agency (U.S. EPA): The federal agency charged with setting policy and guidelines, and carrying out legal mandates for the protection of national interests in environmental resources.

Unload:  The air compressor continues to run, usually at full RPM, but no air is delivered because the inlet is either closed off or modified, not allowing inlet air to be trapped.

Unloaded Horsepower:  The power that is consumed to overcome the frictional losses when operating in an unloaded condition.

Unloading:  The release of contaminate downstream that was initially captured by the filter medium.

Unloader:  A mechanical device used to reduce the capacity of a refrigeration compressor.

Unsaturated Zone:  See Vadose Zone.

Unstable (Reactive): A substance which in the pure state, or as produced or transported, will vigorously polymerize, decompose, condense, or will become self-reactive under conditions or shocks, pressure or temperature.

Unsupported Gloves:  Unlined gloves without any type of fabric lining.

Up Gradient:  The direction from which water flows in an aquifer.

Up Stream:  In the direction against the current of a stream.

Upcycling:  A term coined to describe the creation of a product with higher intrinsic value, manufactured from a material at the end of its service life, which had a lower initial end use value. It is important to note that the term as currently used, does not provide insight into environmental benefit (e.g. there may actually be less environmental benefit to upcycling if energy used to upcycle is more than recycling back to the same product).

Upflow:  A type of air conditioning system that discharges air into the conditioned space via a top-mounted discharge plenum or through an overhead duct system.

Upflow Furnace:  A furnace in which air is drawn in through the sides or bottom and discharged out the top.

Upper-air UVGI: Refers to the use of UV lamps directly in the upper part of a room.

Upper Explosive Limit (UEL):  The highest concentration (expressed in percent vapor or gas in the air by volume) of a substance that will burn or explode when an ignition source is present.

Upper Fixed Point:  The higher of two reference points used to “fix” a temperature scale. Example: The boiling point of water at standard temperature and pressure, is the upper fixed point of a Celsius scale.

Upper Respiratory Tract:   Structures that conduct air into the lungs, including the nasal cavity, mouth, pharynx, and larynx.

Upright Vacuum:  An easy way to define an upright vacuum cleaner is that it is the type that you push rather than pull behind you.  There are a wide variety of different upright designs for many different cleaning situations. Uprights generally have a revolving brush roll to provide agitation and may have one motor that provides the suction and also turns the agitator or it may have two motors, one to provide suction and one to drive the brush. Uprights have traditionally been the favorite type of vacuum cleaner in the U.S. with canisters being more popular in Europe. Recent innovations like on-board attachments and integrated extension hoses and advanced filtration systems have made uprights much more versatile and therefore now able to offer many of the features of canisters.

Uprights: Vertical members supporting the sides of a trench.

UPS:  Uninterruptible Power Supply. An electrical system containing batteries which will provide power to the computer system in the event of a blackout. A continuous UPS is always in the circuit with main power passing through it. A standby UPS is out of the circuit and is switched over in a fraction of a second when needed. In most large data centers, the UPS is only able to support the system for a few minutes until a generator can be started.

Upstream HEPA Filter:  A design in which the HEPA filter is placed before, or upstream of the motor.

Upstream Impacts:  Environmental impacts that are caused by the extraction of raw materials, transportation, and the manufacturing process.

Upstream Side:  The feed side of the filter. Fluid that has not yet entered the filter.

Urban Airshed Model: A three-dimensional photochemical grid model designed to calculate the concentrations of both inert and chemically reactive pollutants in the atmosphere. It simulates the physical and chemical processes that affect pollution concentrations.

Urban Heat Island:  The increase in ambient temperature that occurs in cities because paved areas and buildings absorb more heat from the sun than natural landscape and additional heat is generated by vehicles, lighting, and other equipment.

Urban Ore:  The concept that wastes of today, such as materials disposed of in landfills, may serve as sources of valuable raw materials, such as metals, in the future.

Urea:  Crystalline residue of urine contamination on flooring and sub floor.

Urea-Formaldehyde Binders: A material used in insulation to adhere pressed wood products. Urea-formaldehyde foam insulation is infrequently used because emissions were found to cause a relatively high indoor concentration of formaldehyde.

Urea Formaldehyde from Insulation (UFFI):  A form of insulation no longer in use because of excessive formaldehyde emissions and documented associated health impacts.

Urethane:  Resin used as a sealant.

Urethanes:  A family of plastics (polyurethane’s) used for varnish coatings, foamed insulations, highly durable paints, and rubber goods.

URI (Upper Respiratory Infection): Medical term for a cold.

Urticaria:  Pronunciation (ŭr′ti-kar′i-ă).  An eruption of itching wheals, collquially called hives, usually of systemic origin; it may be due to a state of hypersensitivity to foods or drugs, foci of infection, physical agents (heat, cold, light, friction), or psychic stimuli.  Syn: hives (1), rtication (2).  [L. urtica].
 
U.S.D.A.:  United States Department of Agriculture.

USDOE (The United States Department of Energy): A federal agency whose mission is to advance the national economic and energy security of the United States, to promote
scientific and technological innovation in support of that mission, and to ensure the environmental cleanup of the national nuclear weapons complex.

U.S. Department of Energy (DOE):  Federal agency responsible for conducting energy research and regulating nuclear materials used for weapons production.

U.S. Environmental Protection Agency (EPA):  Federal agency responsible for enforcing federal environmental laws. Although some of this responsibility may be delegated to state and local regulatory agencies, EPA retains oversight authority to ensure protection of human health and the environment.

Useful Life:  Determined when contamination causes a filter or system to have an adverse (lower) flow rate, low efficiency or high differential pressure, providing for an inefficient operation.

Useful Solar Energy: The amount of solar energy contributing to the total heat load. It is expressed in absolute figures (kWh) or per unit collector area (kW/m2).

User Seal Check:  An action conducted by the respirator user to determine if the respirator is properly seated to the face; see negative pressure seal check, positive pressure seal check.

USGBC (United States Green Building Council): A not-for-profit organization that promotes buildings that are environmentally responsible, profitable and healthy places to
live and work.

USGO:  Unit General Safety Officer.
 
U.S.P.:  United States Pharmacopeia/National Formulary: The “Bible” of pharmaceutical manufacturer and test protocol for filtration media using Edition/Title XXI as a basis for evaluation.
 
“Usual” Temperatures (Thermal Comfort): If the adaptation processes are working satisfactorily then we expect people to be comfortable in the environments they are accustomed to, for they will have had adequate opportunity to adjust the temperatures to suit their requirements, and to modify their living patterns to suit the temperatures. It follows that knowledge of these ‘usual’ environments would provide thermal comfort information of a basic kind.

 Utility Air:  See plant air.

Utility Easement: The area of the earth that has electric, gas, or telephone lines. These areas may be owned by the homeowner, but the utility company has the legal right to enter the area as necessary to repair or service the lines.

Utilization Factor:  The ratio in percentage of the time that the equipment is in operation to the total working time.

U-Value:  The amount of heat flowing through a square foot of building materials.

UV:  UV or ultraviolet light is a form of light that has been used for office lighting for many years. Over time, scientists have also found that this form of light has other uses, including uses in disinfection. Ultraviolet light has wavelengths shorter than other forms of light but longer than X-rays.