Formaldehyde (HCHO) is a known human carcinogen and causes a variety of health e

Question

Formaldehyde (HCHO) is a  known human carcinogen and causes a variety of health effects. Formaldehyde can be introduced to indoor air from a new carpet, fiberboard,  and other building materials.  After Hurrican Katrina in 2005, the Federal Emergency Management Agency (FEMA) provided 100,000 trailers and mobile homes for displaced gulf coast residents. Unfortunately, those trailers were built with materials that emitted high levels of formaldehyde. Testing in 2007-2008 found an average indoor formaldehyde of 95 ug/m3 with a range of 4-730 ug/m3 (http://www.cdc.gov/nceh/ehhe/trailerstudy/assessment.htm) Adverse respitory effects are observed when formaldehyde is greater than or equal to 120 ug/m3.

Assume the average trailer volume is 70m3 and the air changes 2.5 per hour (ach). The decay constant for formaldehyde is 0.4/hour and the ambient outdoor concentration is 2 ug/m3

A) calculate steady state emission rate that gives you 95 ug/m3

B) These concentrations were measured when the trailers were 2 years old and the emission rate had significantly decreased by that time. New material rates as high as 290 mg/hr have been reported. Calculate the steady state HCHO concentration (ug/m3) resulting from that emission rate.

C) Compare the answers for threshold adverse respitory effects

Explanation / Answer

Average concentration = 95ug/m3

Let, x be the steady state emission rate in ug/hr

=> x(1hr) + 2.5(2ug/m3)*70-2.5*70*95 = 95*70

=> x = 22.925mg/hr

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Let 'y' be the steady state concentration of trailer,

=> 290000 + (2.5*2*70)-(2.5*y*70) = y*70

=> 3.5*y*70 = 290000 + 350 = 290350

=> y = 1.185 mg/m3

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The threshold concentration is 120ug/hr

=> x(1hr) + 2.5*2*70 -2.5*70*120 = 120*70

=> x = 3.5*120*70 - 2.5*2*70 = 29.05 mg/hr

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