[Brideinneed]

I was at a wedding reception at the weekend. The room had about 200 people in (not sure of the actual size of the room, but it was a squeeze!). There were about 16-18 tables, each with 9 lit tea lights on, which we're giving off quite a bit of heat! The room was also very poorly ventilated and any open doors kept being shut.

My question is, do you think. is it possible to get a bit of carbon monoxide poisoning from this amount of candles in that sort of room? I know it's probably not the sort of chemistry question that would appear on here normally, but I really wanted to know the answer!

[sjb]

I was at a wedding reception at the weekend. The room had about 200 people in (not sure of the actual size of the room, but it was a squeeze!). There were about 16-18 tables, each with 9 lit tea lights on, which we're giving off quite a bit of heat! The room was also very poorly ventilated and any open doors kept being shut.

My question is, do you think. is it possible to get a bit of carbon monoxide poisoning from this amount of candles in that sort of room? I know it's probably not the sort of chemistry question that would appear on here normally, but I really wanted to know the answer!

Probably a bit unlikely, but why not try the maths? How much oxygen would there be in the room? What sort of mass of wax would be burnt?

[Borek]

No idea how much monoxide is produced when candle burns. For sure it produces some mixture of CO₂ and CO, but I have never seen any data on the composition of combustion products. Probably it is highly variable and depends on the type of candle, the way it burns and so on, which makes any measurements difficult to reproduce.

[Corribus]

What a fun problem! I played around with it, got an answer, but I won't post until the OP takes a stab at it.

EDIT: I made an assumption that CO is only produced in significant quantities if there isn't enough O₂ to burn all the available fuel.

[Arkcon]

You used the term, "poorly ventilated", but you don't qualify that in any way.  What range of air exchange rates do you consider likely were happening?  I mean, it was warm and close, not airtight.

[Brideinneed]

Ah thanks for the replies!

I'm not a student of chemistry, last time I did anything chemistry related was back in school in 1997, so I know nothing about this subject!

When I say poorly ventilated, I mean there was no air con, and no fans to blow air around the room. It wasn't air tight, but any doors were kept mostly shut. With 200 people in and the amount of heat coming off the candles, it was rather warm for even me (I've been known to put a cardigan on whilst on holiday in Egypt, and only really ever been too hot whilst visiting the Hoover dam).

I've only been wondering, as I know with open fires in the home, there needs to be sufficient ventilation via the chimney flue thing and people have got poisoned from disposable bbqs in tents before.

[Corribus]

Alright, so here are my thoughts. (I'm going to try to read through my chicken scratch.) In general one way to do a risk assessment is to come up with a worst case scenario, so that's what I'm going to do. Here's how I went about it:

Assumption: CO is only produced in net quantities if and only if there is not enough oxygen to burn all the wax to completion via efficient combustion (to carbon dioxide). I justify this because even when you have 60% of the theoretical oxygen available, you only produce 12% CO by volume in the exhaust, and that number drops quickly to zero as the volume of oxygen increases to the theoretical value and beyond. There is a nice little plot of this here, page six: http://www.mha-net.org/docs/codes/man_combustionanalysisbasics.pdf . So, if we can show that the amount of oxygen in the room far exceeds the amount needed to fully combust all those candles, we can assume no CO is produced.

Assumption: The room is sealed completely. This gives us a very conservative look at the system in question. If there is no air exchange at all, this is a 'worst case scenario'. You didn't give room dimensions, but let's say a table occupies about a 10x10 ft area, ten people a table equals about 2000 square foot room for 200 people (20 tables). I'm going to put the ceiling at 15 ft. That's pretty low for a banquet hall and doesn't allow any room for a dance floor or anything, but again the point here is to be conservative. Call the total volume of space an even 850 cubic meters.

Assumption: The atmosphere is dry (makes calculation easier), meaning there is 20.95% oxygen by volume in the room. We'll assume standard pressure of 1 atm and standard temperature of 298 K. In an 850 cubic meter room, that means there are 178 cubic meters of oxygen (partial volume). At STP, there are 22.4 liters of oxygen per one mole (ideal gas assumption), or a total 3987.2 moles of oxygen in the room.

Assumption: The candles are made of paraffin wax, the chief component of which is C₃₁H₆₄. This substance combusts according to the equation C₃₁H₆₄ + 47O₂ --> 31CO₂ + 32H₂0. I looked up the dimensions of a tea light candle and found they are typically 1.5 inches in diameter and about 16 mm tall. Do a little math and you get a volume of wax 18.145 cubic mm per candle. The density of parrafin is about 0.781 g/cm³, meaning each candle has about 14.17 g of wax. You said 18 tables with 9 candles per table, equals 162 candles, which comes to 2295.54 grams of C₃₁H₆₄ as fuel for the entire room. The molecular weight of C₃₁H₆₄ is 436 grams per mole, meaning there are 5.26 moles of C₃₁H₆₄ fuel to be burned.  Based on the reaction equation, 47 moles of oxygen are consumed to burn one mole of fuel completely, meaning to burn all the candles to completion, 247.22 moles of oxygen are needed. Given that you have 3987 moles of oxygen in the room, burning all of the candles in the room consumes only ~6.2% of the available oxygen.

Based on the assumption that CO production drops off substantially as the amount of oxygen approaches the theoretical amount needed to burn the fuel, and here we have almost 20x the theroetical amount, it seems reasonable to conclude that a negligible amount of CO is produced from those candles. Recall that this is based on the assumption of a completely sealed room, so the true amount of oxygen is considerably higher for a ventilated room. (Even though you say "poorly ventilated", there's probably far more air exchange than you'd think.. otherwise people would suffocate after a few hours.)

Speaking of suffocations - people are also breathing in that air. You could probably factor that in somehow. Like starting with an average tidal volume (amount of volume of air per breath) at about 500 mL, times a fast respiratory rate of 20 breaths per minute (say people are dancing), times 200 people = 120,000 liters of air consumed in an hour.  But there are 850,000 liters of volume of air in the room. After an hour, 200 people have only consumed about 14% of the available air in the room (assuming you don't re-breathe the same air - bad assumption, I know, but what the hell, right? It's still conservative). A tealight burns for about 3 hours or so, right? So during that time you would still have 58% of the available air in the room not breathed in (same caveat), more than enough to fully combust those candles and produce negligible CO. Suffocation is a far bigger concern than CO poisoning.

(You could do this in the reverse way, by the way, starting with a toxicity threshhold of, say, 35 ppm and figuring out what % of the wax needs to convert to CO to induce poisoning, and I think you'll find it's a similarly safe value.)

Anyway, that's all back of the envelope kind of stuff but I think you're within the margins of safety. Most likely if you were feeling light headed, it was because of the heat or stuffiness in the room... or the alcohol.   I'd even say that partially depleted oxygen levels (or high CO₂ levels) would be a better explanation than CO for you feeling not so hot.

The reason people get CO poisoning from camping stoves, cars in garages, woodburning fireplaces and furnaces is because these can be small spaces, the relative ratio of volume of fuel to the volume of space is large, and/or the rate of fuel consumption is fast. Candles burn slowly by design. Gasoline or wood burns quickly. So you have a fast combustion and a small, poorly ventilated space. You consume the available oxygen fairly quickly, which puts you in that danger zone for efficient CO generation. Moreover, incomplete combustion tends to happen when the burn temperature is low (as in smoldering coal or soot) or when a part is damaged that lowers the efficiency of the burn (such as a cracked heat exchanger in a furnace). None of these are problems with burning tealights, which burn efficiently and leave little residue behind to offer a smoldering concern.

OK, so, did I make any math errors in there?  I had notes all over the margins of a scrap piece of paper and I had a hard time reading my handwriting.

[Brideinneed]

Wow, thank you for such an interesting and long answer! That equation was completely over my head, but I understand what you're getting at and I'm really happy to now know for sure. I'd never have been able to work any of that out myself, so thanks to everyone who looked and answered.