Having explored some ironstone mines in the Midlands, I've occasionally experienced bad air/ low oxygen levels.
Ive read that this problem can be dependant on the weather, if so, which weather conditions are likely to maximise the problem?, and are certain times of the year worse than others? Lastly, is there more danger of build ups of Carbon Dioxide etc, in flooded mines, or dry mines?

From experience low atmospheric air pressure conditions tend to bring on the bad air!

wet or dry you'll find bad air!

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Ironstone mines will generally suffer from low oxygen levels as the iron in the rock oxidises to form iron oxide, leaving any non circulating air in the mine depleted in oxygen. The effect of air pressure is to force the bad air further back into the mine at times of high pressure but as the pressure drops the bad air expands into areas which are otherwise ok.
So at times of low atmospheric pressure, typically when its raining outside, you will find gas where you haven't seen it before. Mines with big open worked out areas are most prone to this.
Its very easy to get complacent and think that you know where the gas will be, but take a flame lamp or a detector and trust what it is telling you - this is very dangerous stuff. The voice of experience speaks!

Having spent a great deal of time in ironstone mines in the North-East over the years, we've looked at all sorts of things like temperature, weather, air pressure and wind direction and the one thing thats for sure is there are no "rules" for a given day. Especially if there's more than one entrance to a mine.

But as a VERY general guide for when theres only one way in and out, when the air pressure has been rising for a few days you'll probably have better conditions than when its been falling for the last few days.

But the bottom line with Ironstone is always use a gas meter.

Thanks for the replies so far, especially the advice about low and high air pressure, building up over a number of days!, this’ll give me a better idea of when it’s most practical to go exploring!??

Many years ago we had a very close run thing in Harecastle rail tunnel after entering a side drift down to the disused ochre filled 1st canal tunnel. Cheapest safety option is probably something like this.

https://www.ebay.co.uk/itm/Steel-and-Brass-Eccles-Miners-Lamp-The-protector-Lamp-and-Lighting-Company/173158038583?hash=item285105fc37:g:RIkAAOSwkrFagzdD 

Many years ago we had a very close run thing in Harecastle rail tunnel after entering a side drift down to the disused ochre filled 1st canal tunnel. Cheapest safety option is probably something like this.

https://www.ebay.co.uk/itm/Steel-and-Brass-Eccles-Miners-Lamp-The-protector-Lamp-and-Lighting-Company/173158038583?hash=item285105fc37:g:RIkAAOSwkrFagzdD 

I heard that the person who that happened to had a 4 gas that read v low O2. He didn't believe it & went in anyway!!
I've known that level & its foibles since I was a kid (about 50yrs). It's got some of the best quality blackdamp I've ever come across, I swear you could MIG weld with it. ;D
When it's really bad, you can exhale & see your breath floating on it.
As it's off a large railway tunnel & slightly below floor level, the transition from fresh air to no air is incredibly abrupt.

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I heard that the person who that happened to had a 4 gas that read v low O2. He didn't believe it & went in anyway!!
I've known that level & its foibles since I was a kid (about 50yrs). It's got some of the best quality blackdamp I've ever come across, I swear you could MIG weld with it. ;D
When it's really bad, you can exhale & see your breath floating on it.
As it's off a large railway tunnel & slightly below floor level, the transition from fresh air to no air is incredibly abrupt.

It happened to us on 26/10/90 (from boat cruising logs), no gas detection kit whatsoever and totally unexpected situation.
IIRC, the other incident was very much later.

I heard that the person who that happened to had a 4 gas that read v low O2. He didn't believe it & went in anyway!!
I've known that level & its foibles since I was a kid (about 50yrs). It's got some of the best quality blackdamp I've ever come across, I swear you could MIG weld with it. ;D
When it's really bad, you can exhale & see your breath floating on it.
As it's off a large railway tunnel & slightly below floor level, the transition from fresh air to no air is incredibly abrupt.

It happened to us on 26/10/90 (from boat cruising logs), no gas detection kit whatsoever and totally unexpected situation.
IIRC, the other incident was very much later.

Yes, I think the one to which I referred was about 8 yrs ago.
I can well understand why it was unexpected, that transition is quite unbelievable.
I once demonstrated it with a safety lamp, to a retired miner mate. He swore blind I'd knocked the lamp out, until I re-lit it & let him have a go!!

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Quite strange really as there are images about of a guy canoeing in the old canal tunnel, I'll see if I can dig one up.

Found them, water level was much higher during our visit. I wonder if the tunnel sometimes sumps and blocks the air flow.

http://www.tuesdaynightclub.co.uk/Harecastle/index.html 

Edit: Images from 1979.

When I was a student, one of my contemporaries died in Wheal Jane. In company with an experienced miner, the two of them passed behind a ventilation brattice (a temporary barrier, serving the purpose of directing the ventilation flow along the active working level) and were eventually found dead in bad air, some distance along the level.

These things happen very quickly.

Having had experience of Mount Wellington as a working mine, I wouldn’t enter it in its present state for any reason...

Hi,
Having explored Mount Wellington Mine via the main engine shaft (200 feet), before it was reopened, to adit level, I can state that there was no bad air at the time, even though the adit sumped a long way downstream, and no other shafts were open to improve aeration.
After the mine reopened, the very long upstream adit had dried out, and so with wet-suits on, the going was very hot indeed, which may have been partially due to bad air.
I never experienced noticable bad air anywhere underground, except for in an overpopulated air-bell in Swildon's Hole!
There was also a case of foul gas in Baker's Pit Cave, Devon, which lies beneath a fermenting refuse tip...until a second quarry entrance was blasted into, causing such strong air currents that the system dried out in about one year...
Radioactivity in granite mines has also been recorded, but not extensively monitored, except where uranium was worked.
But smoking in small chambers will rapidly foul up air, causing thick smog...
Generally speaking, columns of warm saturated air rise up out of shafts on cold days, sometimes for many hundreds of feet. This was how cavers noticed the world-record 'Pierre St Martin Aven' where choughs soared in the column above the tiny entrance all surrounded by deep snow.
Cavers have done quite a lot of speleo-meteorology, and that subject is quite well understood. As for mines ?
Best wishes,
D.Send.

Having spent a reasonable amount of time in complex ironstone mines with gas detectors and having seen how the air 'shifts', even to the extent of O2 levels dropping a few percent by the mere action of people walking down a passage towards you, I'd mark down anybody who could claim to be able to predict how the air will react at any given time as an outright fantacist.

We've obviously made such predictions many a time... 😞

Hi Fpop,
All I remember was that the shaft, (heavily timbered) dropped down to the junction of the two main adit levels. The descent was memorable, because we only had 100 feet of ladder, which had to be lowered further down on a rope!
After reopening, the shaft was bored out to a considerable diameter...way below adit level. (At the time, Hugh Browning was looking for the County Adit, and Mt Wellington seemed a possible access point).
Is it possible that some fractured rocks exhale CO2, as in some sedimentary shales, due to degraded methane gas. Relaxation forces around mine workings could perhaps activate such effects? Mining companies should know the answer...
Regards,
D.Send.

Found them, water level was much higher during our visit. I wonder if the tunnel sometimes sumps and blocks the air flow.

http://www.tuesdaynightclub.co.uk/Harecastle/index.html 

Edit: Images from 1979.

That level is a bit of an oddity really. Not entirely convinced that it's a link to the old Brindley canal tunnel (at least not directly). The level with the dreadful air is in the west wall of the rail tunnel - pointing away from the canal & (reputably) 18' above it. It's heading roughly towards Ashwood colliery & seems to pre-date the rail tunnel. If you cross to the east wall of the tunnel, & go a few yards north, you'll find its continuation. It seems to me, to be more likely that the tunnel severed the existing level. It's eastern continuation heads towards the old "Bread & Herring" (Golden Dale) colliery. This pit certainly connected with the Brindley tunnel via a boat level in the Great Row seam. This was told to by my grandfather many years ago. I've since obtained a copy of the Walcot Gibson version of the BGS map of the area, which confirms this.
This topic aint half wondering!

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In general, old ironstone mines are best avoided.
The only time we go into them, we have a big fan at the entrance and that seems to cure any problems.......

That level is a bit of an oddity really. Not entirely convinced that it's a link to the old Brindley canal tunnel (at least not directly). The level with the dreadful air is in the west wall of the rail tunnel - pointing away from the canal & (reputably) 18' above it. It's heading roughly towards Ashwood colliery & seems to pre-date the rail tunnel. If you cross to the east wall of the tunnel, & go a few yards north, you'll find its continuation. It seems to me, to be more likely that the tunnel severed the existing level. It's eastern continuation heads towards the old "Bread & Herring" (Golden Dale) colliery. This pit certainly connected with the Brindley tunnel via a boat level in the Great Row seam. This was told to by my grandfather many years ago. I've since obtained a copy of the Walcot Gibson version of the BGS map of the area, which confirms this.
This topic aint half wondering!

Tend to agree on adit not being parallel to other tunnels. PM sent.