How Functional Units Are Developed and Quantified

As I described in the previous post, the functional unit serves as the reference unit for the study, normalizing all inputs and outputs to it, and enables comparisons between products with similar functions. It is generally expressed in qualitative terms, with sufficient detail to enable the LCA practitioner to convert it into a quantitative number. Oftentimes, life cycle assessments do not document how the functional unit was developed, as described in the presentation by Vee Subramanian and Eric Williams at the ACLCA LCAXII Conference in Tacoma, Washington. Life Cycle Assessments from Ecosystem Analytics Inc. include a thorough explanation on how the function unit was developed, which also provide detail on how the quantitative functional unit is calculated. This adds full transparency to the LCA and enables a thorough review of the basis for the functional unit.

For example, for the LCA Ecosystem Analytics recently completed comparing handkerchief use with facial tissue use, the functional unit was defined as: the number of nose blows per surface area for an average American adult over 1 calendar year, encompassing the use pattern during 4 respiratory illnesses (896 nose blows) and daily use during well periods (337 nose blows). But what is the use pattern over these periods, and how does that relate to the number of handkerchiefs and facial tissues used? This is described in Section 2.1.2.1- Functional Unit Creation, broken up into 3 sections: Nose Blow Frequency, Nose Blows per Facial Tissue or Handkerchief, and Handkerchief Reuse and Laundering.

Nose Blow Frequency

As described in ISO/TR 14049 Technical Report, defining and quantifying the function of a product is an essential part of developing the functional unit. For this LCA, the sole function of both handkerchiefs and facial tissues was to blow one’s nose. But how often does one blow his or her nose? Luckily, medical research on treating respiratory illnesses sheds light on this question. In a study on transmission of cold viruses, study participants blew their nose up to 24 times per person over 12 hours (2 nose blows/hour per person). According to Yale and Liu, the average length of a common cold is 7 days and the average adult has 2 to 4 colds a year. We assumed that the average person sleeps for 8 hours and is awake for 16 hours.

So for one average adult during respiratory colds: 2 nose blows/hour x 16 hours/day x 7 days/cold x 4 colds/year = 896 nose blows/year

But the colds in this scenario only last 28 days (7 day/year x 4 colds/year). What about the 337 other days of the year (365 days/year – 28 sick days/year)? Let’s assume that the average adult blows his or her nose once a day – 337 extra nose blows/year.

Nose Blows per Facial Tissue or Handkerchief

Standard sized paper facial tissue and cloth handkerchiefs differ in size. Handkerchiefs are 4 times as large in surface area. During respiratory illnesses, facial tissue and handkerchiefs are generally used intensively. We based the number of nose blows per tissue or handkerchief based on the relative surface area of the products: 2 nose blows/tissue and 8 nose blows/handkerchief.

• Tissue use during cold: 896 nose blows/yr ÷ 2 nose blows/tissue = 448 tissues/yr
• Handkerchiefs during cold: 896 nose blows/yr ÷ 8 nose blows/handkerchief = 112 handkerchiefs/yr

During everyday use, this LCA assumes that either facial tissue or handkerchiefs are used once.

• Tissue, base use: 337 nose blows ÷ 1 nose blow/tissue = 337 tissues/yr
• Handkerchief, base use: 337 nose blows ÷ 1 nose blow/handkerchief = 337 handkerchiefs/yr

In summary,

• Total tissue use: 448 tissues/yr + 337 tissues/yr = 785 tissues/yr
• Total handkerchief use: 112 handkerchiefs/yr + 337 handkerchiefs/yr = 449 handkerchiefs/yr

Handkerchief Reuse and Laundering

Although facial tissues are disposed after use, cloth handkerchiefs are reused. This LCA assumes that 30 handkerchiefs are in circulation and all 30 will be used and then washed as a fraction of the individual’s total laundry.

• Handkerchief use: 449 handkerchiefs/yr ÷ 30 handkerchiefs in use = 14.97 times washed

In Summary:

Qualitative Functional Unit: the number of nose blows per surface area for an average American adult over 1 calendar year, encompassing the use pattern during 4 respiratory illnesses(896 nose blows) and daily use during well periods (337 nose blows).

Quantitative Functional Unit: 785 tissues/yr or 30 handkerchiefs washed 14.97 times/yr