In the pharmaceutical world, precision and safety are non-negotiable. Labs, pilot plants, manufacturing suites, and cleanrooms all handle potent chemicals and active pharmaceutical ingredients every day. But even the best processes can introduce risk. This may happen especially from short-term exposure peaks of hazardous substances.  

When a reactor operator opens a sealed vessel, even a brief release of vapor can lead to a sudden spike in chemical concentration. This short-term exposure, though lasting only minutes, can still cause severe irritation, respiratory distress, or lasting health effects if it exceeds safe limits. The real danger lies in the fact that traditional monitoring systems focused only on 8-hour averages (TWA: Time-Weighted Average) may overlook these short-lived but hazardous peaks. 

In such cases, the STEL (Short-Term Exposure Limit) compliance becomes essential. In the pharmaceutical industry, failing to manage short-term exposures can lead to: 

  • Acute health incidents among workers 
  • Regulatory noncompliance and fines 
  • Downtime, reputation damage, and liability risks 
  • Reduced trust with regulatory agencies and customers 

So, I will continue reading this article. Here we’ve discussed what STEL is, why it matters in pharma, the challenges companies face, and how to ensure compliance in a practical, effective way. 

 

What is STEL (Short-Term Exposure Limit)? 

STEL, or Short-Term Exposure Limit, refers to the highest concentration of a chemical that workers can safely be exposed to for a short time, usually 15 minutes, without experiencing harmful effects. It complements the 8-hour (TWA) limit. While TWA measures long-term exposure over an entire work shift, STEL focuses on short-term spikes that can still cause irritation, dizziness, or other health problems, even if the overall average looks safe. 

Regulatory agencies set STEL values for many hazardous chemicals to prevent these short bursts of overexposure. For example, OSHA sets a 15-minute STEL of 2 ppm for formaldehyde, compared to its 8-hour TWA of 0.75 ppm.  

(Source: https://www.osha.gov/laws-regs/regulations/standardnumber/1910/1910.1048). Similarly, for benzene, OSHA limits short-term exposure to 5 ppm over 15 minutes, even if the average exposure across the shift remains lower (Source link: https://www.osha.gov/laws-regs/regulations/standardnumber/1910/1910.1028). In short, STEL helps identify and control those sudden peaks in chemical concentration that TWA measurements might miss. 

 

Why is STEL Compliance Critical in the Pharmaceutical Industry? 

  1. Many Intermediates are Potent Even in Small Doses

Pharmaceutical compounds are often biologically active. Even these are designed to act at very low concentrations. This means that even short exposures to intermediates or solvents can have significant health effects (e.g., sensitizers, irritants, or systemic toxins). 

  1. Complex Processes and Variable Loads

Pharma manufacturing often involves operations like granulation, milling, solvent evaporation, pressurization, or transfer of volatile reagents. These steps can cause transient spikes in airborne concentration. In fact, these are exactly the kind of scenarios STEL is meant to guard against. 

  1. Worker Safety & Retention

Short-term chemical exposures, like sudden irritation or breathing trouble, can make workers feel unsafe and anxious. Over time, this not only lowers morale but can also lead to higher employee turnover. Keeping workers protected from these sudden exposure peaks is not just the right thing to do. It’s also essential for maintaining a healthy and stable workforce. 

  1. Regulatory Scrutiny & Liability

Regulators and inspectors expect robust exposure control systems. If an acute exposure event occurs, proper STEL monitoring and controls can be a powerful defense. Without it, you risk fines, shutdowns, or litigation. 

  1. Avoiding Subclinical Damage

Even if chemical exposure stays just below the STEL limit, repeated short-term contact can still cause hidden health effects or make workers more sensitive over time. Following STEL compliance adds an extra layer of protection, helping you maintain safety beyond just meeting average exposure limits. 

 

Common Challenges to Achieving STEL Compliance  

1. Lack of Data for Novel Compounds

For many newly developed APIs or intermediates, there may not be any existing exposure limits or STEL values. In such cases, companies must estimate safe levels using toxicology studies or data from similar chemicals.  

2. Hidden Hotspots Despite Good Ventilation

Even in facilities with strong ventilation systems, certain areas, like near hoods, valves, or sampling points, can have higher chemical concentrations. If you rely only on general area monitoring, these localized spikes might go unnoticed. 

3. Limitations of Monitoring Equipment

To detect short-term exposure spikes, you need fast and accurate sensors. Traditional sampling tools that collect data slowly can miss sudden increases in chemical levels, giving a false sense of safety. 

4. Balancing Safety with Productivity

Sometimes, engineers may hesitate to add stricter safety controls, like stronger ventilation or containment, because it could slow down production or reduce output. This creates an ongoing challenge between maintaining safety and meeting production goals. 

5. Recordkeeping and Compliance Challenge

It’s important to document when and where STEL limits were approached, what safety controls were in place, and how operators responded. Poor or incomplete records can lead to compliance issues during audits. 

 

Best Practices for Ensuring STEL Compliance  

✅Conduct a Detailed Exposure Assessment 

Use real-time monitoring tools like analyzers, photoionization detectors, or infrared sensors to quickly detect short-term exposure spikes. Identify high-risk activities, such as opening valves, taking samples, or transferring materials, and take air samples during those operations. If no STEL data exists for a new compound, use data from a similar chemical with known toxic effects as a reference point. 

 

✅Focus on Engineering Controls First

Design processes to keep chemicals contained, such as using closed systems or glove boxes during transfers. Install local exhaust ventilation (LEV) near points where emissions could occur. Automate containment and purging systems to control exposure during sampling or venting. 

 

✅Improve Administrative Controls and Work Practices

Limit how long and how often workers perform tasks that can cause exposure peaks. Plan the work so high-risk steps happen when fewer people are around. Develop clear standard operating procedures (SOPs) that make safe work practices automatic and easy to follow. 

 

✅Use Personal Protective Equipment (PPE) as Backup Protection

Even with good controls in place, workers may still need PPE such as respirators or protective hoods during short-term exposures. However, PPE should be the last line of defense and not the main method of protection. 

 

✅Add Real-Time Alerts and Safety Controls

Set up sensors that trigger alarms or automatically shut down equipment if chemical levels exceed STEL limits. Connect these alerts to your process control system so workers are immediately informed and can take quick action. 

 

✅Train, Monitor, and Continuously Review

Educate workers on what STEL means and how to respond when alarms go off. Regularly review your safety controls—process changes, new materials, or scale-ups can change exposure patterns. Keep detailed records of monitoring data, safety incidents, corrective actions, and system updates to ensure ongoing compliance. 

 

Metrics & KPIs you should Track 

To maintain STEL compliance and ensure continuous safety improvement, it’s essential to track specific performance indicators. These metrics help you identify exposure risks early, measure control effectiveness, and improve your overall safety strategy. Here’s what to focus on: 

  • Spot Trends and Weak Points:  

Tracking data like STEL exceedances or alarm rates helps you identify recurring issues in processes, equipment, or work practices. 

  • Measure Safety System Performance:  

Monitoring how often and how quickly your controls respond to exposure spikes shows whether your safety systems are truly effective. 

  • Support Compliance and Audits:  

Having detailed metrics makes it easier to demonstrate compliance during internal or external safety audits. 

  • Prioritize Improvements:  

By comparing different KPIs (like high-risk tasks or frequent alarms), you can focus resources on areas that need immediate attention. 

  • Promote Accountability and Awareness:  

Sharing these results with teams reinforces the importance of STEL control and helps build a proactive safety culture. 

 

Conclusion 

In the pharmaceutical industry, controlling average exposure (TWA) is necessary—but it’s not enough. Peak exposures can cause acute harm, erode confidence, and attract regulatory scrutiny. That’s why STEL compliance is a critical pillar of any mature safety program. 

If your organization is scaling, changing processes, or simply revalidating existing safety systems, make STEL compliance a priority. Ensure your exposure assessments, controls, sensors, SDS, and training align with STEL limits—not just average values.