91Ó°ÊÓ

Air in industrial plants is subject to contamination by many different chemicals, and companies must monitor ambient levels of hazardous species to be sure they are below limits specified by the National Institute for Occupational Safety and Health (NIOSH). In personal breathing-zone sampling (as opposed to area sampling), workers wear devices that periodically collect air samples less than 10 inches away from their noses. Breathing-zone sampling and analysis methods for hundreds of species are set forth in the NIOSH Manual of Analytical Methods. \(^{13}\) For benzene, NIOSH specifies a recommended exposure limit (REL) of 0.1 ppm time-weighted average exposure (TWA), and the Occupational Safety and Health Administration (OSHA) permissible exposure limit (PEL) is 1.0ppm TWA. A worker in a petrolcum refinery has a personal breathing-zone sampler for benzenc clipped to her shirt collar. Following the NIOSH prescription, air is pumped through the sampler at a rate of \(0.200 \mathrm{L} / \mathrm{min}\) by a small battery-operated pump attached to the worker's belt. The sampler contains an adsorbent that removes essentially all of the benzene from the air passing through it. After several hours, the sampler is removed and sent to a lab for analysis, and the worker puts on a fresh sampler. On a particular day when the temperature is \(21^{\circ} \mathrm{C}\) and barometric pressure is \(730 \mathrm{mm}\) Hg, samples are collected during a 4-h period before lunch and a 3.5-h period after lunch. The analytical laboratory reports \(0.17 \mathrm{mg}\) of benzene in the first sample and \(0.23 \mathrm{mg}\) in the second. (a) Calculate the average benzene concentration, \(C_{\mathrm{B}}(\mathrm{ppm}),\) in the worker's breathing zone during each sampling period, where 1 ppm = 1 mol C \(_{6} \mathrm{H}_{6} / 10^{6}\) mol air. (b) The worker's TWA is the average concentration of benzene in her breathing zone during the eight hours of her shift. It is calculated by multiplying \(C_{\mathrm{B}}\) in each sampling period by the time of that period, summing the products over all periods during the shift, and dividing by the total time of the shift. Assume that the worker's exposure during the unsampled 30 minutes was zero, and calculate her TWA. (c) If the worker's exposure is above the recommended limits, what actions might the company take?

Step by step solution

01

Calculation of average benzene concentration for each period

First, we need to convert the mass of benzene into the number of moles. We know the molecular weight of benzene (\(C_6H_6\)) is 78.11 g/mol, so we use this to convert the milligrams into moles. Next, we calculate the total air volume this sample was taken from using the pump rate and the time period. Lastly, we find the concentration by dividing the number of moles of benzene by the number of moles of air. We can find the number of moles of air by using the ideal gas law n = PV/RT, where we will use the given temperature and pressure and R as the ideal gas constant in appropriate units.

02

Calculation of the Time-Weighted Average (TWA)

We need to find the average benzene concentration over her 8 hour shift. As per the problem, during the un-sampled 30 minutes, benzene exposure was zero. Therefore, the TWA is calculated by adding the product of the concentration and the time for each period and then dividing the total by the total time of the shift.

03

Analyze and Recommend Actions

Now we need to compare the calculated TWA to the NIOSH recommended exposure limit (REL) and the Occupational Safety and Health Administration (OSHA) permissible exposure limit (PEL). If our calculated TWA exceeds these limits, we need to make some recommendations. This could include improving the ventilation system, providing respirators or masks, or changing work procedures to reduce exposure. These are general suggestions and the specific actions may depend on the exact situation in the plant.

Unlock Step-by-Step Solutions & Ace Your Exams!

• Full Textbook Solutions

  Get detailed explanations and key concepts

• Unlimited Al creation

  Al flashcards, explanations, exams and more...

• Ads-free access

  To over 500 millions flashcards

• Money-back guarantee

  We refund you if you fail your exam.

Over 30 million students worldwide already upgrade their learning with 91Ó°ÊÓ!

Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Air Contamination

Air contamination in industrial settings is a significant concern as it involves the presence of harmful substances in the air that workers may breathe. These contaminants can include a variety of chemicals, often varying based on the specific type of industry. Contaminated air can have adverse health effects, ranging from mild irritation to serious respiratory diseases.
The National Institute for Occupational Safety and Health (NIOSH) establishes limits to ensure that workers are not exposed to concentrations that could be detrimental to their health. Monitoring and managing air contamination is crucial for maintaining a safe working environment. Regular assessments and appropriate safety measures, like ventilation and personal protective equipment, help minimize risks.

Benzene Exposure

Benzene is a volatile chemical compound commonly found in industrial environments, particularly those dealing with petroleum and chemicals. It is colorless and highly flammable. Prolonged exposure to benzene is a health risk as it is known to be carcinogenic.
NIOSH sets a recommended exposure limit (REL) for benzene to be 0.1 parts per million (ppm) as a time-weighted average (TWA). Meanwhile, the Occupational Safety and Health Administration (OSHA) limits exposure to 1.0 ppm. These limits are in place to protect workers from potential health effects like leukemia, a type of cancer, which has been linked to long-term benzene exposure.

• Continuous monitoring through sampling ensures that exposure levels do not exceed these limits.
• Employers should mitigate benzene exposure by controlling these levels, enforcing protective measures, and educating workers.

Personal Breathing-Zone Sampling

Personal breathing-zone sampling is a method used to assess the exposure of workers to airborne contaminants. This approach involves attaching a sampling device to the worker, usually within 10 inches of their nose, to collect air samples.
The device typically works by drawing air through a filter or absorbent, capturing contaminants like benzene. The collected sample is then analyzed in a laboratory to determine the concentration of harmful substances.

• This type of sampling provides an accurate representation of an individual worker's exposure, compared to ambient sampling which measures the general air quality of a work area.
• It enables companies to take specific actions if any individual's exposure exceeds recommended levels.

Time-Weighted Average (TWA)

The Time-Weighted Average (TWA) is a calculation used to assess the average level of exposure to contaminants over a period, typically the duration of a work shift. This average considers the exposure levels during sampled times and estimates for unsampled periods.
In practice, TWA involves:

• Multiplying the concentration of a contaminant for each sampled period by the time duration of the period.
• Summing these products.
• Dividing the total by the total hours of the work shift.

It is a standard method to determine compliance with exposure limits like those set by NIOSH and OSHA. TWA helps in understanding the potential risk levels workers face in their work environment throughout their shift, ensuring protective measures are based on realistic exposure assessments.