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Four days ago, I experienced the worst indoor air quality I’ve measured in London. Not from wildfires, not from Bonfire Night, but from traffic stuck outside my window.

The Setup: Roadworks on the Main Road

Last Monday morning, I woke up to the sound of jackhammers. The council had started emergency roadworks on the main road—apparently a water main burst overnight. Fair enough, these things happen.

What I didn’t anticipate was what would happen to the air quality in my ground-floor flat on a normally quiet residential side street.

The Traffic Diversion Disaster

By 8 AM, the diversion signs were up. Every car, van, lorry, and bus that normally used the main road was now funnelling through our narrow residential street. And I mean every vehicle.

My street, which usually sees maybe 20 cars an hour, suddenly had a constant stream of stop-start traffic. Engines idling. Diesel fumes. The lot.

I had my Temtop M10 sitting on the windowsill (I’d been monitoring outdoor air for a different project). The readings told the story.

The Numbers: Worse Than I Expected

Normal day (main road open):

• Outdoor PM2.5: 8-15 μg/m³
• Indoor PM2.5: 5-10 μg/m³
• Comfortable, no issues

Monday with diversions (8 AM - 6 PM):

• Outdoor PM2.5: Peak of 78 μg/m³
• Average outdoor: 45-55 μg/m³
• Indoor (windows closed): 18-28 μg/m³
• Indoor (windows cracked): 35-42 μg/m³

For context, WHO guidelines recommend 24-hour average PM2.5 below 15 μg/m³. We were hitting triple that outside my window.

Why Side Streets Are Worse

Here’s what I didn’t realize until I watched the traffic for an hour: side streets create worse air quality than main roads for several reasons:

1. Constant Stop-Start Main roads have traffic lights and flow. Side streets have parked cars, tight corners, and constant braking. Every time a vehicle stops and accelerates, it produces more emissions than steady driving.

2. Canyon Effect My street is narrow with buildings on both sides. The pollution gets trapped between the buildings rather than dispersing. It’s like a pollution canyon.

3. Diesel Concentration When the diversion started, we got everything—delivery vans, buses, lorries. Normally these stick to main roads. Suddenly they’re all idling outside residential windows.

4. No Air Circulation Main roads are wider with better airflow. Side streets? The air just sits there, especially on a still day like Monday was.

The Indoor Air Problem

I keep my windows closed most of the time anyway (ground floor, security concerns), but I usually crack them open for ventilation in the morning. Not this week.

Even with windows closed, indoor PM2.5 climbed from my usual 5-10 μg/m³ to 18-28 μg/m³. Air infiltrates through gaps, doors opening, ventilation systems. You can’t completely seal a flat.

With windows even slightly open? Indoor readings matched outdoor within 20 minutes. Completely unusable.

The Health Impact I Noticed

By Tuesday evening, I had:

• Scratchy throat
• Slight headache
• Generally feeling a bit rubbish

Nothing dramatic, but definitely noticeable. I’m generally healthy with no respiratory issues, so this was unusual for me.

My neighbour with asthma? She was using her inhaler more than usual and eventually went to stay with family until the roadworks finished.

What I Did About It

Immediate actions:

1. Closed all windows - obvious but necessary
2. Ran my air purifier - HEPA filter, kept it on high
3. Avoided outdoor exercise - normally run in the mornings, skipped it
4. Worked from a café - got out of the flat during peak traffic hours

Air purifier effectiveness: With the purifier running on high, I got indoor PM2.5 down to 12-15 μg/m³. Not perfect, but much better than 28 μg/m³.

The purifier was noticeably working harder than usual—I could hear it ramping up whenever the traffic got particularly bad outside.

The Bigger London Problem

This experience made me think about London’s air quality more broadly. We focus on main roads and congestion zones, but what about residential streets used as rat runs?

Some observations:

• Waze and Google Maps route drivers through residential streets to avoid main road delays
• Roadworks create temporary but severe air quality problems for residents
• School streets (closed to traffic during drop-off/pick-up) exist for good reason
• Low Traffic Neighbourhoods are controversial but might actually help air quality

The irony? The main road probably has better air quality than our side street did during the diversion. At least traffic flows there.

When Did It End?

The roadworks took four days. By Friday afternoon, traffic was back on the main road and my street returned to normal.

Friday evening readings:

• Outdoor PM2.5: Back to 10-14 μg/m³
• Indoor PM2.5: Back to 6-9 μg/m³
• Breathing: Much easier

The difference was immediate and obvious. I opened the windows Friday evening for the first time in four days.

What I Learned

1. Location matters more than you think Living on a side street is usually quieter and better for air quality. But during diversions or when it becomes a rat run, it can be significantly worse than main roads.

2. Air purifiers actually work I was somewhat skeptical about how much difference a home air purifier makes. During this week, it made a very noticeable difference to indoor air quality.

3. You can’t always control outdoor air But you can control indoor air with closed windows, purifiers, and strategic timing of ventilation.

4. Traffic patterns affect health This wasn’t abstract statistics—I felt physically worse during those four days. For people with existing respiratory conditions, the impact would be much more serious.

5. Urban planning decisions have real consequences Where traffic flows, how diversions are managed, and which streets become shortcuts all directly affect residents’ health.

The Uncomfortable Truth

London’s air quality has improved significantly over the past decade. Congestion charges, ULEZ, cleaner buses—all helping. But we still have a long way to go.

And when infrastructure fails (burst water mains, roadworks, accidents), the backup plan often involves pushing traffic through residential streets where people live, sleep, and breathe.

My four days of bad air were temporary and manageable. But for people living on permanent rat runs or near major junctions? This is their daily reality.

Practical Advice If This Happens to You

If your street becomes a temporary diversion route:

1. Close windows during peak traffic (usually 7-9 AM and 5-7 PM)
2. Use an air purifier if you have one—it genuinely helps
3. Ventilate strategically - early morning or late evening when traffic is lighter
4. Monitor air quality - knowing the numbers helps you make decisions
5. Consider staying elsewhere if you have respiratory issues and it’s going to be long-term

If you’re planning where to live in London:

• Check traffic patterns at different times of day
• Look for streets that aren’t obvious shortcuts
• Consider upper floors if air quality is a concern (pollution concentrates at ground level)
• Check if the area has planned roadworks or infrastructure projects

Final Thoughts

I’m back to normal now. Windows open, air purifier on low, outdoor PM2.5 around 12 μg/m³. The roadworks are done, traffic is flowing on the main road, and my side street is quiet again.

But I won’t forget those four days. It was a stark reminder that urban air quality isn’t just about averages and long-term trends—it’s about what you’re breathing right now, outside your window, in your home.

And sometimes, the quietest residential street can temporarily become the worst place for air quality in the neighborhood.

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